CC BY
33
UDC 678.06:678.5
A. A. Olkhov, M. A. Goldshtrakh, V. S. Markin,
R. Yu. Kosenko, Yu. N. Zernova, G. E. Zaikov, A. L. Iordanskii
INFLUENCE OF POLYHYDROXYBUTYRATE ON PROPERTIES OF COMPOSITION FILMS ON THE BASIS HYDROPHOBIC AND HYDROPHILIC POLYMERS
Keywords: polyhydroxybutyrate, mechanical characteristics, polyethylene, polymer blends, water permeability, polyvinyl alcohol,
hydrophobic and hydrophilic polymers.
Mechanical characteristics and water permeability of films on the basis of mixtures of LDPE - PHB and PVA - PHB depending on their composition were investigated. It was shown that the additive of PHB increments water permeability of a polymeric matrix, but at major dosage of PHB the permeability of films can be reduced because of ability of PHB to fix water. It was determined by DSC, polarization IR- spectroscopy, WAXS and SAXS- experiments that there is a connection between structural architecture of films at crystalline ( or at molecular ) level with water permeability and mechanical performances. It was shown that the decreasing of strength of films occurs at concentration PHB above than 10 % (for LDPE - PHB mixtures) and above than 20 % (for PVA - PHB mixtures).
Ключевые слова: полигидроксибутират, механические характеристики, полиэтилен, полимерные смеси, водопроницаемость,
поливиниловый спирт, гидрофобные и гидрофильные полимеры.
Исследованы механические характеристики и водопроницаемость пленок на основе смесей ПЭВД - ПГБ и ПВС - ПГБ в зависимости от их состава. Показано, что добавление ПГБ увеличивает водопроницаемость полимерной матрицы, но при большой концентрации ПГБ проницаемость пленок может быть снижена из-за способности ПГБ фиксировать воду. Методами ДСК, поляризационной ИК-спектроскопии, WAXS и SAXS установлена связь между структурой пленок на кристаллическом (или на молекулярном) уровне с водопроницаемостью и механическими характеристиками. Показано, что снижение прочности пленок происходит при концентрации ПГБ выше 10% (для смесей ПЭВД - ПГБ) и выше 20% (для смесей ПВС - ПГБ).
Introduction
The blending of semicrystalline biodegradable and friendly environmental thermoplastic such as bacterial poly(3-hydroxybutyrate) [PHB] with one of the cheapest packing or industrial polymer such as low density polyethylene [PELD] and with hydrophilic polyvinylalcohole [PVA] is a perspective tool to obtain novel materials with combined characteristics of the origin components along with economic advantages for material performance.
To improve the mechanical behavior of PHB and simultaneously to lower the cost of its production the modification can be made through the PHB blending with other relevant polymers. Resulting polymer blends are potentially able to gain the properties different from the ones of parent blend-forming polymers [1-4].
Poly(3-hydroxybutyrate) (PHB) was used as the modifying polymeric component. The selection of this polymer is due to its biocompatibility with animal tissues and blood. Taking into account similar properties of PVA, one may expect that a new class of polymer materials for medical purposes will be created [5, 6].
A widespread procedure for regulating the drug release rate involves controlled changes in the balance of hydrophilic interactions in the polymer matrix at the molecular level. Therefore, regulation of structural organization at the molecular and supramolecular levels makes it possible to control the rate of drug delivery and hence to improve the therapeutic efficacy of new medicines.
Experimental
The fine powdered PHB were provided by Biomer Co (Krailling, Germany). The chemical structure of PHB structure is well-established in the literature [7] and the viscosity-average molecular weight Mw = 2.5 105 was determined by intrinsic viscosimetry in chloroform solution. The granulated LDPE is commercial product of SAFCT Russian designated standart (15803-020) with following characteristics: Mw = 2.5 105 and specific density 0.92 g/cm3.
All blends were made by melting in a singlescrew extruder (ARP-20). Preliminary mixed compositions with different ratio components (PHB/LDPE): 2/98, 4/96, 8/92, 16/84, and 32/68 were loaded in the extruder. Temperature in a ringing heater head did not exceed 185 oC and the frequency of the screw rotation was 100 turns/min. The factor of blowing (2.0) and the drawing ratio (5.0) have controlled the production of the blend films with 40-50 ^m thickness. The unblended LDPE and PHB were also processes under identical extrusion conditions to undergo a thermal history similar to the history for the blends.
The study is concerned with PVA 8/27 Russian trade mark and PHB Biomer Krailing Germany Lot M-0997. The residual acetate group concentration and Na acetate salt concentration in PVA comprise 8.2 and 0.04% wt. respectively. Molecular weight of PVA is 64 000g/mol. with melting point equals 146oC.
The loaded concentration of PVA:PHB ingredients varied as 100:0, 90:10, 80:20, 70:30, 50:50, and 0:100. The blends were produced using a single screw extruder, ARP-20 with L/D = 25, diameter = 0.20 cm. Electricity heating was used to obtain 180 oC flat extrusion profile. The components were first premixed in Brabender Plasticorder PCE330 at 170 oC and at 60 rpm rotor speed after drying the ingredients in an air oven at
101 oC for 8 h. The screw rotation was 100 rpm. The films obtained with final thickness 60 mkm allowed to air cool to room temperature.
A study was made of water permeability (in two-compartment cell) and mechanical properties for the binary blends at varying blending ratios (0, 2, 4, 8, 16, and 32 wt % PHB in LDPE as well as 0, 10, 20, 30 and 50 wt % in PVA). Films of the blends were obtained with the single screw extruder. The structure of blend films was characterized by various techniques including differential scanning calorymetry (DSC), wide angle X-ray scattering (WAXS), and infra red fourier transmission spectrometry (FTIRS). Mechanical measurements (ultimate tensile strength) were performed under ambient temperature by an Instron Tensile Tester.
The orientation in LDPE and PHB components was monitored separately by Fourier transform infrared (FTIR) dichroism measurements in range 700 - 1500 cm-1. In a conventional manner, IR dichroism (DR) was recorded from the absorbance of appropriate bands (at 729 cm-1 for CH2 groups of LDPE crystallites [8] and at 1228 cm-1 for CH2 groups of PHB crystallites [9]) with radiation polarized parallel or normally to the extrusion direction. From the corresponding absorbances (An and A±) a dichroic ratio (DR) can be estimated as DR = An/A±. A wire grid polarizer was used as instrument of IR polarization.
Scanning electron microscopy (SEM) has been used to characterize the morphology generated by melt extrusion. The circular extruded polymer blends were immersed in liquid nitrogen and then fractured. The fractured surfaces were coated with fine-dispersed Au. The areas of blends were viewed end-on by scanning electron microscope ''Tesla BS 301'' at magnification from 100X to 10 000X.
Characteristics of the films are studied by DSC technique with Metler PR4000 calorimeter at heating rate 20 o/min, wide angle x-ray scattering method (WAXS) at two different directions: parallel and normally to film surface. The tensile modulus and elongation at break of the films were determined from measurements on an «Instron 1122» tensile apparatus. The drawing speed was 1.0 cm/min and the results were averaged at least five tests.
Permeation of water vapor was measured at 23 oC using a regular two-compartment cell especially designated for PHB films. The relative humidity in feed compartment is maintained constantly at 90%. Water content in registration compartment was very close to zero. Amounts of water transferred through polymer films are determined by weighting of KOH as absorber of water. The deviation of 5 parallel measurements for each experimental point is averaged 0.85%. The sensing device was accurate to ±0.0001 g at 23oC .
Results and discussion
Fig. 1 shows that the tensile strength over the range 0-32 wt% PHB has the appearance of a curve with maximum followed by the minimum. The curve can be separated onto two fields: the first part in the range 0-16 wt. % PHB has the maximum near 8 wt.%.
%, PHB
Fig. 1 - Dependence of ultimate tensile on the blend compositions for films of PVA/PHB and LDPE/PHB respectively
The ascending branch of this maximum is due to the increasing of segmental orientation and formation of crystalline texture (WAXS data). The descending branch may be attributed to the decrease of segmental order (SAXS data). The following rise of the tensile strength (between 16 and 32 wt%) is due to the incorporation of component (PHB) with the higher modulus.
} Endo
106.4
176.0
Fig. 2 - Typical DSC termograms (fragments) of PHB/PELD blends (32 wt % PHB)
Two separate melting temperatures of the PELD-rich (Tm1) and PHB-rich (Tm2) phases are observed over all blend compositions, 2 - 32 wt% PHB. Values of Tm1 and Tm2 remain constant for a series of the polymer blends in range 4 -32 wt%. But positions of two peaks on the termograms are displaced from melting points of homopolymers about 1.5 - 3o below. These results demonstrate immiscibility of PHB/PELD blends in above concentration range. Thermal properties of LDPE/PHB blended films by table 1.
In the initial interval (0-20%PHB) of PVA/PHB blends the stable tensile strength is shown in Fig.1. After 20% concentration this parameter is sharply decreased due to transition from compatible to incompatible system. The incompatible matrices are characterized by imperfection of crystalline organization.
Table 1 - Thermal properties of LDPE/PHB blended films
centration interval (0-20%) the blend permeability mono-tonically decreased with PHB concentration, see Fig 4.
Blend content, % PE/ PHB Melting point Т 0С 1 m.? ^ Melting heat, A H m, J/g
PE PHB PE PHB
100/0 107,0 - 70,0 -
98/2 106,4 172,0 58,0 58,0
96/4 106,3 172,6 55,0 38,0
92/8 106,5 171,8 70,0 44,0
84/16 106,8 173,2 60,0 40,0
68/32 105,9 173,2 35,0 48,0
0/100 - 175,4 - 61,9
Blend content, % PE/ PHB Crystallinity Х , % (*) Crystallisation temperature 0C
PE PHB PE
100/0 40 - 89,6
98/2 35 42 89,8
96/4 29 45,4 89,4
92/8 30,5 47 89,5
84/16 28 49 89,7
68/32 25 55 89,5
0/100 - 68,8 85,0 + 78,6
(*) -DSC data.
PE
• 2%. PHB
A 4%. PHB
8%. PHB
16% PHB
32%. PHB
-Ж- PHB
200 300 t, h
Fig. 3 - The quantity of water (g), diffused through PE/PHB blend films
At the low PHB concentrations, the water flux resistance is maximal and exceeds the permeability of LDPE film, see Fig.3. Then, the water permeability is increased with the PHB concentration. It is significant that the same manners have both the dichroism ratio and mechanical tensile strength dependence on the PHB concentration for the blended films. It is apparent from the above data that transport behavior of water in the blended films is substantially affected by the orientation of polymer segments (Fi) and defects in inter phase PHB/PE area.
From DSC data the good compatibility of blends for hydrophobic (PHB) and hydrophilic (PVA) polymers is observed until 30 wt % of PVA. In this con-
—■— PVA
• 10% PHB
—A— 20% PHB
ф 30% PHB
—1-- 50% PHB
E
< 1,0 -
full
dissolvation
150 t , h
Fig. 4 - The quantity of water (g), diffused through PVA/PHB blend films
In spite of the crystallinity decrease in PVA, the total water diffusion is decreased also due to interaction of hydroxyl groups with carbonyl groups of PHB and hence due to water solubility depression. For both PE/PHB and PVA/PHB blends the specific inflection points are shown. The initial stage of permeability reveals the relaxation of elements of blend structure on the molecular and crystalline levels.
The transformation of the amorphous fields has to effect such important characteristics as permeability and diffusivity of water. The transport processes proceed exclusively in amorphous part of any blend matrix and hence this process will be structure-sensitive relative to change of structural organization in inter crystalline fields.
Kinetic curves of vapor water permeation through the blend films at different ration PHB/PVA presented in Fig.4. Contrary to permeability through parent PVA films, all permeability curves through blends have three specific range corresponding to three different ways of water diffusion.
The initial range features the low rate of water permeation where diffusion is conjugated with immobilization of water molecules on functional groups of PHB (ester groups) [10] and more intensively on hydroxyl groups of PVA [11]. In this temporal interval of time the transitional flux of water takes place that is typically for all hydrophilic polymers. The next intermediate range (II) determines the quasi steady-state regime of transport, where water diffusion is complicated by residual structural relaxation more typical for PHB molecules. A rise in vapor water permeation is dictated by both the increase of free volume ratio and segmental mobility in the blends. The last two effects result from screening the functional groups by absorbed water molecules [12] as well as redistribution of hydrogen bonds in the blends as response on water affect [13]. The rupture of hydrogen bonds formed initially between ester (PHB) and hydroxyl (PVA) or between two hydroxyl groups as the effective crosslinks promotes swelling in the blends and as consequence an increase of both water diffusivity and water equilibrium sorption. The third range of permeability curves can be
2,5
2,0
rn 1,5
0,0
recognized due to the inflection point observed for all samples containing PHB. It seems likely that to this moment the structural relaxation is completed and the water transport proceeds in accordance with regular diffusion mechanism [14].
The results of DSC scans for PHB-PVA blends and the parent polymers which were prepared by extrusion at various proportions of the components are given in Table 2.
Table 2 - Characteristics of the composite films based on PVA and PHB
PVA: PHB, % Tm, K *PW'10 и, [g-cm/cm2-h-Pa ] **Cw-10J, [g/cm3-Pa]
100:0 402/ 443 2,1 0,28
90:10 396/ 459 0,56 0,068
80:20 403/ 443 0,79 0,023
70:30 405/ 448 0,86 0,0075
50:50 405/ 451 0,94 0,01
0:100 449 0.0025 7,5- 10-4
Notes: two melting temperatures (Tn) of the composite films
As was found, the positions of the high-temperature and low-temperature peaks in the DSC curves, which correspond to the melting points of the starting components, are almost independent of the blend composition and remain invariable in the whole concentration range under study (Table 2). However, the transient region characterizing the glass transition temperature of the PVA-PHB system assumes different positions on the temperature axis depending on the concentration of PHB. This situation is vividly illustrated in Fig. 5, where Tg of the blend is seen to increase with the content of PHB.
T , oK
330 r g
■
320 -
■
310 -
■
300 - " ■
290 -1-1-1-1-1-
0 20 40 60 80 100
[ PHB] , %
Fig. 5 - Dependence glass temperature from concentration of PHB in PVA/PHB blended films
shift of Tg can reflect the tendency for miscibility of the components. In detail, thermo physical characteristic analysis will be presented in our forthcoming paper [16].
Along with thermophysical data and the transparence in PHB-PVA films observed at 0 - 30 % concentration interval, the findings of x-ray (WAXS) method show that each of the components is capable of forming the own crystalline phase. Analysis of diffractograms (Fig.6) allows extracting in general spectra the reflexes which pertain to individual crystalline phases of PHB and PVA simultaneously.
5. nnr1
Fig. 6 - X-ray diffractograms of the films with a composition of 80:20 (a) and 70:30 (b) wt % recorded alone the orientation axis (1) and at the angle of 90° (2) or 20° (3) to the orientation axis. S = 2sin0/A, where 0 is the X-ray scattering angle and k is the wavelength
At all proportion of the components in the blends, PHB conserves the elementary cell parameters a = 0.576 nm, b = 1,32 nm, and c = 0.596 nm which correspond to orthorombic elementary cell [17]. The PVA reflexes are typical for quasi crystalline modification (the gamma-form) constructed by parallel-oriented mac-romolecules in dense packaging [18]. On the diffractogram reflex at S = 2.21 nm- corresponds to the own phase of PVA.
WAXS measurements were taken at the different orientation of film position relative to X-ray irradiation beam. For all samples the diffractograms reveal the existence of axial cylindrical texture in PVA. The axis of texture coincides with extrusion direction and hence the PVA molecules in quasicrystalline fields oriented along extrusion direction. In samples with 10 and 20 % wt of PHB a well-defined axial texture of PHB crystallites is evidence where the texture axis coincides with direction of extrusion as well. However, the PHB crystallites oriented relative to the texture axes so that the extrusion direction in line with the axes of elementary crystalline cell. Hence, the axes of PHB molecules are normally oriented relative to extrusion direction.
The low-temperature transition between 24 and 54oC may be related to the glass transition temperatures (Tg) for both parent polymers (for PHB is 24.1 oC and PVA is 53.9 oC) and polymer segments of these polymers interacting in blends, see [15]. This perceptible
Diffractograms of the samples containing 30 and 50 %wt of PHB show that the most part of crystalline phase in PHB is isotropic, without texture and only residual amount of oriented and textured crystallites is present in 30%wt- PHB-contained sample.
These findings allow concluding that in the PHB-PVA blends at 30 %wt PHB content the structural transition from textured to isotropic crystalline state occurs. Such transition could be attributed to phase inversion of polymer matrix taking place in the same concentration range, about 30 wt% of PHB. It is common knowledge that in the range of phase inversion both crystalline and physical properties of polymer blends are changed, see e.g. [19]. In this work we have studied the effect of structural inversion on mechanical behavior of the blends at different concentrations of PHB. The drastic decrease observes on the curves tensile strength - concentration (Fig. 1) in the same concentration interval. Besides, on the curve reflecting the dependence of elastic modulus on PHB concentration there is the minimum located in the same concentration interval near 30 % wt where the phase inversion proceeds. At the low PHB concentrations in the blends, their behavior at rupture is preferably determined by the mechanical properties of PVA while at the PHB concentration more than 30 %wt these characteristics are closely analogous to the behavior of PHB matrix. The results presented in Figures 1 and 6 do not contradict the physical concept of phase inversion involving both crystalline fields and intercrystalline (amorphous) fields in the blends.
Besides, we can not exclude the leakage of water flux through defect zones formed on the border between two components just as it happened in PHB-PELD blends described in our work. The sharp build-up of heterogeneity in PHB-PVA blend films in 30 - 50 %wt interval sets one assuming an intricate mechanism of water transport including both the proper diffusion and the transport through porous areas formed of structural elements of the blended components. In this situation we has to treat these coefficients only as effective transport coefficients.
Conclusion
In perspective, the heterogeneous LDPE/PHB blends and homogeneous PVA/PHB blends represents the novel biodegradable materials. The variation of PHB concentration in the blended films permits to regulate the special morphology and as result their water barrier properties. On the one hand, due to the fibrile morphology the novel films are superior to the origin polymer films (PHB, LDPE, PVA) in some mechanical characteristics and water flux resistance. On the other hand, the formation of such compositions enhances the rate of degradation under climatic wet conditions.
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© A. A. Olkhov - Ph.D., Senior Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, aolkhov72@yandex.ru; M. A Goldshtrakh - Ph.D., Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, V. S. Markin - Ph.D., Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, R. Yu. Kosenko - Ph.D., Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, Yu. N. Zernova - Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, KNRTU, Kazan, Russia, ov_stoyanov@mail.ru; A. L. Iordanskii - Doctor of Chemistry, Head of Diffusion Phenomena in Polymer Systems Laboratory, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia.
© А. А. Ольхов - канд. техн. наук, ст. науч. сотр., Институт химической физики им. Н.Н. Семенова РАН, Москва, Россия, aolkhov72@)yandex.ru; М. А. Гольдштрах - канд. хим. наук, науч. сотрудник того ин-та; В. С. Маркин - канд. хим. наук, науч. сотрудник того ин-та; Р. Ю. Косенко - канд. хим. наук, науч. сотрудник того ин-та; Россия, Ю. Н. Зернова - канд. хим. наук, науч. сотрудник того ин-та; Г. Е. Заиков - д-р хим. наук, проф. каф. технологии пластических масс, КНИТУ, Казань, Россия, ov stoyanov@)mail.ru; А. Л. Иорданский - д-р хим. наук, зав. лаб. Диффузионных явлений в полимерных системах, Институт химической физики им. Н.Н. Семенова РАН, Москва, Россия.
