CC BY
22
УДК 541.64:547.172:542.943
S. G. Karpova, Yu. A. Naumova, L. R. Lyusova, E. L. Khmeleva, A. A. Popov, G. E. Zaikov
STRUCTURAL-DYNAMIC CHARACTERISTICS OF THE NONWOVEN MATERIALS AND FILMS BASED ON POLYURETHANE, THE STYRENE-ACRYLONITRILE COPOLYMER,
AND THEIR BLENDS
Keywords: biodegradable composition, crystallinity probe EPR, DSC, XRD, ozone, the correlation time, diffusion, nonwoven material,
styrene-acrylonitrile copolymers, molecular mobility.
ESR method and DSC calorimetry method were applied for investigation films and nonwoven materials. For PU and styrene-acrylonitrile copolymers fibers it is shown that a solvent weakly affects the molecular dynamics of chains in the film and nonwoven polyurethane materials and has a strong effect on the molecular mobility in the films based on the styrene-acrylonitrile copolymers and blends with a high content of the copolymer. For the nonwoven material, this effect is insignificant. Breaks at the melting temperatures of mesomorphic structures in PU- styrene-acrylonitrile blends were revealed from the dependence of correlation time on temperature and polymer ratio. The influence of ozone on the amorphous phase of this polymers was studied and it was shown by the spin-probe method that for PU-based films and nonwoven materials ozone has no effect on the molecular dynamics, where as for blends and styrene-acrylonitrile copolymer considerable changes in t take place.
Ключевые слова: биоразлагаемая композиция, исследование кристалличности методами ЭПР, ДСК, РСА, озон, время корреляции, диффузия, нетканый материал, сополимеры стирола и акрилонитрила, молекулярная подвижность.
Для исследования пленок и нетканых материалов использовались методы ЭПР и ДСК. Для волокон из ПУ и ак-рилонитрил-стирольных сополимеров показано, что растворитель слабо влияет на молекулярную подвижность цепей в пленке и нетканых полиуретановых материалах и сильно влияет на молекулярную подвижность в пленках на основе сополимеров стирола и акрилонитрила и смесей с высоким содержанием сополимера. Для нетканого материала этот эффект незначителен. Экстремальные минимумы температур плавления жидкокристаллических структур в смесях ПУ-акрилонитрил-стирол были выявлены из зависимости времени корреляции от температуры и соотношения полимеров. Исследовано влияние озона на аморфную фазу этих полимеров и методом спинового зонда показано, что для пленок на основе ПУ и нетканых материалов озон не оказывает никакого влияния на молекулярную подвижность, в то время как для смесей и сополимера стирола с акрилонитрилом имеют место значительные изменения т.
Introduction
An efficient way to solve problems related to the synthesis of materials with an improved complex of manufacturing and performance properties is to use blends of existing polymers. Choosing the type and ratio of components in the polymer blend makes it possible to vary the structures and properties of blend composites in a wide range. Therefore, the pre-paring polymer blends and designing new materials on their basis is a mainstream direction in the modern technology of polymer processing [1, 2].
The method of polymer processing and the choice of parameters of manufacturing processes are important aspects that determine the structures and properties of polymeric materials. The transfer of polymers to the viscous-flow state via preparation of their solutions is widely used in the manufacture of fibers and threads, paintwork materials, glues, sealants, etc. [3-6].
It is known [1, 3-6] that the choice of sol-vent in the technology of processing of polymer solutions is determined not only by its dissolving power but also by its effect on the structures and properties of the resulting materials. The structuring in solution depends on the nature of the used solvent. Note that the differences in the structures of the materials are largely associated with the differences in the thermo-dynamic qualities of solvents; their interaction with polymer macromolecules; and, as a consequence, the differences
in interactions between macromolecules [1, 5-10]. For the ternary systems solvent-polymer 1-polymer 2, the role of the solvent is even much more complicated.
Along with the thermodynamic quality of a solvent, the structure of polymeric materials obtained from solutions is strongly affected by the volatility of a solvent and the conditions of its removal.
As a continuation of our studies [11] devoted to the effect of solvents on the structures and properties of polymeric materials obtained via processing of polymer solutions, comparative studies of films and fibrous nonwoven materials based on thermoplastic PU, the styrene-acrylonitrile copolymer, and their blends are presented below. The purpose of this analysis is to gain insight into the effects of the conditions and apparatuses of polymer processing and the type of solvent on the structures and proper-ties of the resulting materials.
The electrospinning of fibers is a dry method of fiber spinning where the nature of the solvent controls the theological behavior of the spinning solution and the effective viscosity of the solution in turn determines the energy costs of the process and the diameter and morphology of fibers [12].
At present, ultrathin fibers and items formed on their basis enjoy wide application in biomedicine, cellular engineering, separation and filtration processes, the creation of reinforced composites, electronics, analytics, sensor diagnostics, and a number of other innovative areas [12-16]. The choice of the polymers is associated with the use of PU in light industry, the glue-
and-sealant industry [6, 10, 17], and medicine [18, 19], while the styrene-acrylonitrile copolymer is used in the manufacture of composite filtering and analytical materials [12]. As was indicated in [10, 16], materials obtained via solution processing of the polymer blends PU-styrene-acrylonitrile copolymer, i.e., glue compositions and filtering materials, may find a wider application field owing to the improvement of their performance characteristics relative to those of materials based on individual polymers.
In addition, the choice of the blend compo-sites based on PU and the styrene-acrylonitrile copolymer is associated with the interest of the authors in the compatibility of the polymers and the role of the solvent in this problem.
At present, the structure and properties of PU [18-20], as well as blend composites based on PU and a number of polymers of various families (chitosan [21], polyoxybutyrate [22], and the styrene-acrylonitrile copolymer [23-27]), have been studied.
The compatibility of materials based on the blends of thermoplastic PU of various brands and styrene-acrylonitrile copolymers that were obtained via processing of polymer melts was investigated, and a limited range of ratios at which the mentioned polymers were mutually soluble was observed [23-27]. At a mass ratio of 30:70, the thermoplastic PU and the styrene-acrylonitrile copolymer are incompatible [24-26]. As was noted in [25], the styrene-acrylonitrile copolymer shows a higher solubility in the phase enriched with PU than PU shows in the phase enriched with the styrene-acrylonitrile copolymer.
The DSC and DMA studies of the phase states of the blends for films formed from soluions of PU and the styrene-acrylonitrile copolymer with the use of low-molecular-mass liquids of various chemical classes [11] are in conflict with the above-mentioned data. Therefore, the comparative analysis of the effects of solvents on the structures of both films and nonwoven materials obtained via the electrospinning of fibers is of certain interest for fundamental polymer science and use in practice.
Experimental
The objects of research were dilute and concentrated solutions, films, and nonwoven materials based on the Desmocoll 400 thermoplastic PU (Bayer, Mw = 1.0 105), the SAN 350N styrene-acrylonitrile copolymer (Kumho, Mw = 1.0 105), and their blends at PU-to-styrene-acrylonitrile mass ratios of 10:90, 30:70, 50:50, and 80:20 for the films and 25:75, 50:50, and 75:25 for the nonwoven materials.
The viscosities of dilute solutions were measured on a VPZh-2 Ostwald viscometer [7, 10] in organic solvents of various chemical classes: ethyl acetate, THF, and acetone. The solution concentrations were in the range 0.1-2.0 g/100 mL.
The above solvents were selected on the basis of their dissolving abilities with respect to PU, the styrene-acrylonitrile copolymer, and their blends in the range of concentrations of the used spinning solutions
[10] and on the basis of the setup of the manufacturing process of defect-free fibers via electrospinning.
The films were cast in Petri dishes from concentrated polymer solutions (a concentration of 10 wt %) followed by full removal of the solvent at constant temperature and humidity. The film thicknesses were 300-350 ^m. Full removal of the solvent was ensured via evacuation, and the films were checked via measurements of their thicknesses and masses.
The tested polymeric nonwoven materials were prepared via electrospinning from solutions of PU, the styrene-acrylonitrile copolymer, and their blends; the mean diameters of fibers were 1-10 ^m. The concentrations of spinning solutions were 8-12 wt %. The samples had unit-area masses of 20-70 g/m2 and aerodynamic-drag values of 3-30 Pa at an air-flow velocity of 1 cm/s. The nonwoven materials were prepared with the aid of the Nanospider technology. This is a patented technology of capillary-free highvoltage electrospinning of fibers with the free surface of a liquid [13-15]. The electrospinning process was performed under the following conditions: a temperature of 20°C and a relative air humidity of 60%. The distance to the pick-up coil was 0.2-0.3 m.
The calorimetric studies of the materials we-re performed on a DTAS-1300 thermal analyzer in the temperature range from -90 to +140°C. The heating rate was 20 K/min. The temperature-measurement precision was ±0.5°C
The molecular mobility was studied via the paramagnetic-probe method. The stable nitroxile radical TEMPO at a concentration of 103-104 mol/L was used as a probe. The radical was incorporated into the film and nonwoven materials with small amounts of the styrene-acrylonitrile copolymer from vapor at 40°C and into the samples with high contents of the styrene-acrylonitrile copolymer at 70°C. ESR spectra were recorded in the absence of saturation; this condition was verified by the dependence of the signal intensity on the microwave-field power. The correlation times of probe rotation, t, were calculated from the ESR spectra according to the following formula [28]:
where AH+ is the width of the low-field component of the spectrum and I+/L is the ratio of the intensities of the low- and high-field components, respectively. The error of the measurement of t was ±5%.
Results and Discussion
The viscosity of polymer solutions
Structuring in solutions is determined by the type of used solvent. In this case, the different structures of the resulting materials are primarily related to different interactions of the polymer with the solvent, that is, to different thermodynamic affinities between the solvent and the solute [1, 5, 7, 10].
In this study, the thermodynamic qualities of the solvents with respect to PU, the styrene-acrylonitrile
copolymer, and their blends were quantitatively estimated through the determination of intrinsic viscosities [n] of polymer solutions and the Huggins constants. It is believed [7] that the differences in the intrinsic viscosities of solutions of flexible-chain polymers are associated with the fact that, in different solvents, the sizes of molecular coils are different. In solvents that are good from the thermodynamic viewpoint, coils swell to a higher extent than that in poor solvents; as a consequence, the intrinsic viscosities of dilute solutions in good solvents are higher.
With consideration for the data shown in Fig. 1 and following the fundamental ideas of the physical chemistry of polymers [7], it was shown that, for the polyurethane thermoplastic and the styrene-acrylonitrile copolymer, THF is a good solvent in terms of thermodynamics. For every polymer, the solvents may be arranged in the order of their decreasing thermodynamic quality as follows: THF, ethyl acetate, acetone. A similar picture is observed throughout the range of compositions of the binary blends of PU and the styrene-acrylonitrile copolymer.
InLdL/g
Eii[h:il]Ty of (ïkItitie, J/£
tOOh
Ctt
Copolymer, %
Fig. 1 - Effects of the type of solvent and the PU to styrene-acrylonitrile polymer mass ratio on [n]. Solvents: (1) THF, (2) ethyl acetate, and (3) acetone
The structures of films and nonwoven materials based on blends of PU and the styrene-acrylonitrile copolymer that were formed in THF and acetone were studied via DSC (Fig. 2).
As was shown in [11], mesomorphic structures are formed in the films. These structures were described in detail, for example, in [29]. The character of change in the fractions and melting temperatures of mesomorphic structures with the compositions of the blends for the films and nonwoven materials are different.
Here, the data on change in the fraction of mesomorphic structures in a film that were reported in [11] and the corresponding data obtained in this study for the nonwoven materials are compared. The fraction of mesomorphic structures was normalized to the content of PU in the blend.
Enllinlpy l '1 п. : ! : ri: I «lift)
(I 411
?ll «I
Fig. 2 - Plots of (a, b) the enthalpy of melting and (c, d) the temperature of melting of mesomorphic structures vs. blend composition for (a, b) films and (c, d) nonwoven materials: (1) THF and (2) acetone
Polymer crystallinity
DSC studies (Tables 1, 2) showed that the fractions of mesomorphic structures, a, in PU-based films and nonwoven PU materials are similar and amount to 45-47 J/g (the enthalpy of melting). If, in the
Table 1 - Effect of polymer ratio (wt %) and solvent type on Tg and Tm (°C), as estimated via DSC, for films
Solvent PU Mass ratio of PU to styrene-acrylonitrile copolymer Co poly mer
80:20 50:50
T A m T T A m T J-a T m T a T a
THF 60,3 -35 57 -30 - 50 80
Ethyl acetate 54,0 - 53 -20 48 - 85
Acetone 54,0 -40 48 - 56 - 115
Solvent PU Mass ratio of PU to styrene-acrylonitrile copolymer Co poly mer
T m T 30:70 10:90
T T a T m T a T a
THF 60,3 -35 51,4 55 30 80
Ethyl acetate 54,0 - 65,0 56 80 77 85
Acetone 54,0 -40 55,0 - 76 70 115
case of the films, the addition of a small amount of the styrene-acrylonitrile copolymer leads to a marked
reduction in the fraction of mesomorphic structures (Fig. 2a), then, for the nonwoven materials these changes are not so pronounced (Fig. 2b). At a PU-to-styrene-acrylonitrile mass ratio of 50:50, no mesomorphic structures are observed in the films, while for the fibers, the fraction of these structures is extremely small. At a higher content of the styrene-acrylonitrile copolymer in a blend (above 50%), the fraction of mesomorphic structures in a film increases abruptly, whereas for a nonwoven material, the value of a remains practically the same, regardless of the type of solvent.
Thus, the dependences of a on blend composition for the films and fibers follow different pat-terns, but all of them have breaks corresponding to the 50% content of PU in the blend. This outcome apparently may be explained by phase inversion. In our opinion, the different patterns of the dependences of a on blend composition for the films and nonwoven materials are associated with different types of structural organization of the blends.
Table 2 - Effect of polymer ratio (wt %) and solvent type on Tg and Tm (°C), as estimated via DSC, for fibers
Solvent PU Mass ratio of PU to styrene-acrylonitrile copolymer Copolymer
75: 25 50: 50 25: 75
E 1— CT 1— E 1— CT 1— E 1— CT 1— E 1— CT 1— CT 1-
THF T oo" 0 i OO о - <o <N 95 and -35 as m "Л "Л ©
Acetone 4S "Л OO m ■ as 1 T m" I 0 О 5 О
In addition, the characters of change in melting temperature Tm with the composition of the blend in the films and nonwoven materials for the studied polymers are different. If the melting temperatures for the films and the nonwoven materials decrease with an increase in the fraction of the styrene-acrylonitrile copolymer in the blend (up to 50% in the composite), then, at its higher content, the melting temperatures for the films increase; in contrast, for the nonwoven materials, the melting temperatures decrease more abruptly. It is important that, for both films and nonwoven materials, the character of change in the melting temperature does not depend on the type of the solvent. In addition, these relationships provide evidence that phase inversion occurs at a PU-to-styrene-acrylonitrile mass ratio of
50:50 for both films and nonwoven materials. The values of Tm and a change in a symbate manner. For example, after addition of a small amount of the styrene-acrylonitrile copolymer to the blend, defects in the mesomorphic structures of PU appear in the films; as a result, the values of a and Tm decrease. As to the fibers, a remains practically the same in most cases (the exception being the 50:50 blend). At this composition of the blend, phase inversion occurs and a marked entanglement of chains hampers the formation of mesomorphic structures. Therefore, practically no mesomorphic structures appear in the films formed in THF, while in the case of fibers, the fraction of mesomorphic structures is negligibly small. In blend composites, when the styrene-acrylonitrile copolymer becomes the continuous phase, more and more perfect mesomorphic structures of PU form and, accordingly, the values of a and Tm increase. For a nonwoven material (probably because of the high rate of spinning), the structure of the polymer has no time to transition to the equilibrium state. It is important that, for the films, the values of Tm are higher than those for the nonwoven material. For example, if for the films based on the 30:70 blend of PU and the styrene-acrylonitrile copolymer, Tm=51.4°C, then for the nonwoven material, Tm=39°C. For the PU-based films, the melting temperature is 60.3°C, and for the nonwoven material, the melting temperature is 48.5°C. (The data on other blends prepared in THF are similar.) This result additionally suggests that the mesomorphic structures formed in the nonwoven material are farther from perfect than those in the film samples.
Note that, for the films based on the styrene-acrylonitrile copolymer, the glass-transition temperature is 80°C, while for the nonwoven material, this value is 110°C. For the films of the blends, no glass transition of the styrene-acrylonitrile copolymer occurs in the studied temperature range (from -90 to +140°C) except that in the 50:50 blend of PU and the styrene-acrylonitrile copolymer, whereas for the nonwoven material, the glass transition of the copolymer is observed at PU-to-styrene-acrylonitrile mass ratios of 25:75 and 50:50. This outcome testifies that the amorphous phase in this material is more rigid. The absence of the glass transition in the blend composites other than those mentioned above indicates that PU exerts a plasticizing effect on the structures of the blends and that the compatibility of these polymers is high.
Thus, casting of the films from solutions is a slow process; the system occurs in the equilibrium state; and the addition of the styrene-acrylonitrile copolymer (in small amounts) to PU decreases the fraction of mesomorphic structures of PU owing to the interaction between PU and copolymer molecules, as was shown in [5]. At a higher content of the styrene-acrylonitrile copolymer (above 50%), when PU is a discrete phase, disseminations of PU with a dense surface of straightened chains form. The fraction of the mesomorphic structures of PU in a film material increases at a high content of the styrene-acrylonitrile copolymer because of a lower entanglement of chains in the discrete phase than that in the continuous phase. It
appears that, in the fibers, the structures of the samples have no time to transition to the equilibrium state, because of the high rate of formation. Therefore, an extremely small fraction of mesomorphic structures of PU form in the blend composites of a nonwoven material that contains a high amount of PU (Fig. 2a).
Molecular mobility in polymers
The complex character of change in probe mobility was considered in terms of the model of the binary distribution of segments in less and more dense intercrystalline regions responsible for faster and slower rotations of the TEMPO radical. The study of molecular dynamics in the films and nonwoven materials based on blends of PU and the styrene-acrylonitrile copolymer revealed heterogeneity of the amorphous phase except in the case of PU (Fig. 3). Heterogeneity of the amorphous phase indicates that it contains structures with low (I+) and high (I.) mobility. In what follows, correlation times were calculated for the fast component solely.
The study of the type of solvent showed that the solvent exerts a strong effect on both the time of rotational mobility of probes, t, and, hence, the molecular mobility of polymer chains (Fig. 4). As is seen from Fig. 4a, the highest mobility of the probe is ob-served for the films formed in THF, while the lowest mobility of the probe is detected for the films formed in acetone. In the nonwoven material, the solvent has no marked effect on molecular mobility (Fig. 4b).
3280
3321) 33i(!
Magnetic field, C.J
32»!
34(111
332(1 1360
Magnetic lieEil■ G
Fig. 3 - ESR spectra of (a) the film and (b) the nonwoven material based on the 50:50 blend of PU and styrene-acrylonitrile formed in THF as a solvent
Fig. 4 - Changes in t in (1) acetone, (2) ethyl acetate, and (3) THF in (a) films and (b) nonwoven materials of the blend composition
The addition of the styrene-acrylonitrile copolymer to the polymer matrix entails an increase in t. In the case of the films, as the amount of the copolymer in the composite is increased to 50%, the correlation time changes insignificantly, and only at a higher content of the polymer, when the styrene-acrylonitrile copolymer is a continuous phase, does t increase abruptly. This circumstance suggests an in-crease in the rigidity of amorphous regions. The observed pattern of the dependence of t on the blend composition in the films may be explained by the structure of interfacial regions (Fig. 5). The results of ESR studies were treated and quantitatively interpreted with the use of software from Bruker. With the aid of this software, the molecular mobility in the interfacial regions of the composites were calculated. As is clear from Fig. 5a, in the case of the films, the densest interfacial layers form in the blends containing a small amount of the copolymer. The permeability of the radical across these layers is apparently hindered; therefore, the values of t change slightly after the addition of the styrene-acrylonitrile copolymer (to 50%). In the blends with high amounts of the copolymer, the interfacial interlayer has a lower t value, a result that suggests a higher looseness and, as a consequence, a higher permeability to the radical.
The interfacial interlayer in the fibers, regardless of composition, has a small value of t, a circumstance that suggests a high permeability of the interlayer to the radical, and manifests itself on the dependences of t on composition (Fig. 4b). For both the films and fibers, the dependences of t on composition feature a break at a PU-to-copolymer mass ratio of 50:50. This result confirms the above finding that phase inversion occurs at this ratio of the copolymers.
Note that correlation times t for PU samples formed in different solvents differ insignificantly for both the films and the nonwoven material. At the same time, for the films based on the styrene-acrylonitrile copolymer, these differences are considerable. Thus, the solvent changes the structure of the amorphous regions of the styrene-acrylonitrile copolymer to a much higher extent than that of PU.
< I»1".:
1Ч.Г. & PU. "i.
Fig. 5 - Changes in т in interfacial layers for (a) films and (b) nonwoven materials in (1) acetone, (2) ethyl acetate, and (3) THF vs. blend composition
As is known [5], for this blend composite, the solvents may be arranged in terms of their decreasing thermodynamic quality as follows: THF, ethyl acetate, acetone. In poor solvents, the degree of interaction between the polymers is much higher than that in good solvents and solvents of different qualities create different polymer structures in solution that are preserved after the removal of solvents. The difference in the interactions of the polymers in a solution leads to a difference in the properties of the films and nonwoven fibrous materials prepared from a solution, because structures existing in a solution are partially preserved in these films and nonwoven fibers.
From the thermodynamic viewpoint, the only difference between good and poor solvents is that they interact with the polymer in a different manner; there fore, interactions between macromolecules are likewise different. A higher convolution of macromolecular coils in a poor solvent facilitates an increase in the number of contacts not only between similar macromolecules but also between dissimilar macromolecules [1, 9]. Hence, the solvent has a strong effect on the structures and molecular mobility of the polymers. In the best solvent, THF, the structures of the polymer composites are close to equilibrium. As poorer solvents are used, the state of the structures becomes more and more nonequilibrium; the density of chains increases; and, as a result, the rigidity of macromolecules increases in the sequence of the polymers formed in THF, ethyl acetate, and acetone (Fig. 4a). In the case of fibers, the influence of the solvent on the amorphous structure of the polymer makes itself evident much weaker (Fig. 4b).
The fraction of macromolecules that form dense amorphous regions in the composite films increases (an estimate was performed on the basis of I+/I. , Fig. 5) on passage from good solvent to poor solvent, as evidenced by the data shown in Fig. 6a. An increase in the fraction of PU in the blend leads to a reduction in the fraction of regions with hindered motion, and there is only a single spectrum for the blends containing a small amount of the styrene-acrylonitrile copolymer (less than 50%). For the fibers, these differences are less distinct (Fig. 6b); however, the double spectrum is additionally observed for the composites with high
contents of PU. Note also that the break on the above dependences is likewise observed at a PU-to styrene-acrylonitrile mass ratio of 50:50 (Fig. 6b).
Ü4
pu.
Fig. 6 - Plots of I /I vs. blend composition in (1) acetone, (2) ethyl acetate, and (3) THF for (a) films and (b) nonwoven materials
The solvent determines the concentrations of the radical in the studied polymers. An increase in the density of the amorphous phase in PU (as well as in the composites) formed in various solvents (Fig. 7) is attested by a decrease in the concentration of the radical on passage from the good solvent to a poor solvent for both blend films and fibers. However, if the concentration of the radical in composites formed, for example, in THF is analyzed, changes in the films may be explained by both increases in the densities of the films on passage from PU to the styrene-acrylonitrile copolymer and decreases in the fractions of the amorphous phase accessible to the radical; therefore, the concentrations of the radical in the samples decline (Fig. 7a).
: I О-1, ' H; :.1 : '
; 111', mol/cm3
-1.5
3.(1
IS
in
41)
pu. ж
311
pu. %
Fig. 7 - Variations in the concentration of the radical in (1) acetone, (2) ethyl acetate, and (3) THF in (a) films and (b) nonwoven materials of the blend composition
Another character of the dependences is observed for the PU-copolymer fibers (Fig. 7b). The composition of the fiber does not exert such a marked effect on the concentration of the radical as that in the films. Only at a PU-to-copolymer mass ratio of 50:50 is there a jump in the concentration of the radical on its composition dependences. This outcome suggests that the packing of chains in these fibers is the loosest. It is important that there is an extremum point at this ratio, and this circumstance confirms the finding that phase inversion occurs at this ratio of the polymers.
In addition, the temperature dependences of correlation times for blend films and fibers were investigated. It was found that there is a break at temperatures of 45-55°C that is apparently related to the unfreezing of mesomorphic structures (Fig. 2). The linear dependence is observed only for films based on the 50:50 PU-to-copolymer blend, and exactly in these composites there are no mesomorphic structures. (In the fibers, the fraction of these structures is negligibly small.)
The variation in activation energy with the composition of the polymer blend depends on the type of solvent and the method of preparing polymeric materials (Fig. 8).
Calculation of the activation energies for the rotational mobility of the probe in the studied polymers showed that, for the PU films, the effect of a solvent on Еа is small and the addition of the styrene-acrylonitrile copolymer (to 50%) entails a decrease in the activation energy, a result that is probably related to a decrease in the density of the composites (Fig. 8a). At a higher content of the copolymer in the blend, changes in Еа are not so pronounced, regardless of the solvent type. Note that the activation energy increases from the good solvent to a poor solvent.
In the case of fibers, a change in the activation energy follows other regularities (Fig. 8b). Like parameter a (Fig. 2b), the values of Еа for the blend (up to 50% PU) are practically constant. At a 50% content of PU, the value of Еа decreases abruptly, and in the composites with higher contents of the styrene-acrylonitrile copolymer, the activation energies increase slightly.
£.,. Id/mol
E,. kl/mol
Sil 1 III) PU.
It should be emphasized that the values of Еа for fibrous polymers formed in various solvents other than acetone are similar, whereas for the films, these values differ appreciably (Fig. 8a). It is vital to note that, for all blends except the 50:50 blend, there is a break on the temperature dependences of correlation time in the range 45-55°C. Because, at the same temperatures, there is the peak of melting of the mesomorphic structures, the break on the temperature dependences of т may be explained by the unfreezing of such structures. A gain in Еа at temperatures above 50°C may be as-sociated with a higher activation energy precisely in mesomorphic structures.
Ozonation polymers
The effect of the oxidizer ozone on the amorphous phase of these polymers was investigated with the use of the microprobe method. For example, regardless of the method of preparing composites, polymers containing high amounts of the styrene-acrylonitrile copolymer are subject to oxidation. PU and the blends with small amounts of the copolymer are more resistant to the action of the aggressive medium of ozone (Fig. 9).
I CI 3CI
Time, h
Fig. 8 - Plots of Ea vs. blend composition in (1) acetone, (2) ethyl acetate, and (3) THF in (a) films and (b) nonwoven materials
Fig. 9 - Variation in т with the time of ozonation for (a) films and (b) nonwoven materials. (a) PU-to-copolymer mass ratios of (1)
The correlation times change insignificantly for PU and the 80:20 blend of PU and the styrene-acrylonitrile copolymer during ozonation of the films and nonwoven materials formed in THF. For the composites containing high amounts of the styrene-acrylonitrile copolymer, the values of т increase at low ozonation times (to 2 h) for both the films and the nonwoven materials. Further ozone oxidation leads to decreases in т, and these changes are the most pronounced for the 25:75 PU-copolymer blend and the 30:70 PU-copolymer film. It is known that chemical and physical crosslinking of macromolecules and degradation of chains occurs during oxidation. Chemical crosslinking implies the formation of new entanglements as a result of branches that appear during ozone oxidation. At the initial step of oxidation, crosslinking processes prevail; therefore, the molecular mobility decelerates and, as a consequence, т increases. For a longer effect of ozone, the processes of chain
degradation begin to prevail and this tendency manifests itself as a decrease in т. Hence, it may be inferred that the styrene-acrylonitrile copolymer is the most prone to ozone oxidation. Similar dependences were obtained for the samples formed in acetone and ethyl acetate.
A comparison of the properties of the films and nonwoven materials made it possible to analyze change in molecular mobility of polymer molecules at the early stages of their interaction with oxidative aggressive media.
In addition, a change in ratio I+/I. with an increase in the time of ozonation for the fibers formed in THF was studied. As is seen from Fig. 10, at the initial step of ozonation (before 2 h), this ratio first increases and then decreases.
On
t\n\
0 5 10 \5 (I 5 Ml IS
Time, h Time, h
Fig. 10 - Plot of I1+/I2. vs. time of ozonation for (a) films and (b) nonwoven materials. (a) PU-to-copolymer mass ratios of (1) 30:70 and (2) 50:50; (b) PU-to-copolymer mass ratios of (1) 25:75, (2) 50:50, and (3) 75:25
An increase in I+ U_ is evidence that the fraction of dense amorphous regions increases apparently owing to the physical crosslinking of macromolecules during ozonation; a decrease in this parameter suggests that the destruction of these regions occurs. Correlation time and I+/I. change in a symbate manner. This fact confirms the finding that crosslinking between chains prevails at the initial stage of ozone oxidation.
Conclusion
Thus, it has been shown that differences in the structures of the resulting materials are primarily related to differences in the thermodynamic qualities of the solvents. The type of solvent has the most pronounced effect on the structures and molecular dynamics in the case of the styrene-acrylonitrile copolymer and the blend composites containing high amounts of the copolymer. In this case, mesomorphic structures form in both films and nonwoven materials. At a 50:50 mass ratio of PU and the styrene-acrylonitrile copolymer, there are breaks on the dependences of т, Еа, the concentration of the radical, and the fraction of mesomorphic structures on the blend composition for
both films and nonwoven materials as a result of phase inversion. With the use of the spin-probe method, it has been found that the oxidizer ozone affects amorphous phases in the films and nonwoven materials. It has been shown that, regardless of the method of blend formation, the polymers with high amounts of the styrene-acrylonitrile copolymer are subject to oxidation. PU and the blends with low amounts of the copolymer are more resistant to the aggressive medium of ozone. If, for a film material, the type of solvent shows a strong effect on the molecular mobility in samples, then, in a nonwoven material, this effect is much less distinct. In addition, it is shown that, if the concentrations of the radical in the films decrease by almost an order of magnitude on passage from PU to the styrene-acrylonitrile copolymer, then, in a nonwoven material, this change is insignificant.
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© S. G. Karpova - Ph.D., Researcher of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, Yu. A. Naumova - Ph.D., Researcher of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, L. R. Lyusova - Doctor of Engineering, Full Professor, Researcher of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, E. L. Khmeleva - Researcher of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, A. A. Popov - Doctor of Chemistry, Full Professor, Deputy Director of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, [email protected].
© С. Г. Карпова - кандидат химических наук, сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Ю. А. Наумова - кандидат технических наук, сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Л.Р. Люсова - доктор технических наук, профессор, сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Е. Л. Хмелева - сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, А. А. Попов - доктор химических наук, профессор, заместитель директора Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Г. Е. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, [email protected].
