ARTICLE pubs.acs.org/IECR
Preparation and Rheological Behaviors of Thermoplastic Poly(vinyl alcohol) Modified by Lactic Acid Jing Ding, Si-Chong Chen,* Xiu-Li Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan) College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China ABSTRACT: A thermoplastic modified poly(vinyl alcohol) (PVA-g-LA) with improved melt processing property was prepared by melt polycondensation of PVA with lactic acid. Rheological behaviors of PVA-g-LA copolymers were investigated using a highpressure capillary rheometer. The influences of the content of oligo-poly (lactic acid) graft chains and content of PVA-0588 on the rheological properties of PVA-g-LA copolymers were researched. The rheological parameters including non-Newtonian index and the viscous flow activation energy were analyzed and evaluated. As predicted, the PVA-g-LA samples exhibited noticeable shear thinning behavior. The apparent viscosity, the viscous flow activation energy, and the non-Newtonian index of PVA-g-LA showed obvious correlation with the PLA grafting chain content and PVA-0588 content at a constant temperature and shear rate. Both the increase of the content of oligo-PLA graft chains and content of PVA-0588 with PVA-1799 can improve the melting flow processing of PVA-g-LA copolymer.
1. INTRODUCTION Research and development of poly(vinyl alcohol) (PVA) for various applications has been the subject of great scientific and commercial interest. PVA is a synthetic polymer that is applied in different areas such as textile sizing and finishing agent, emulsifier, photosensitive coating, and adhesives for paper, wood, textiles, and leather.1,2 Moreover, it is a biologically friendly polymer due to its full biodegradability and biocompatibility.3 These properties are, however, counterbalanced by the poor thermal characteristic4 of PVA, which renders melt processing difficult. It is difficult to produce the film through melt processing because the melting temperature is too close to decomposition temperature of PVA. It is a limit for expanding into the market of food packaging application due to high manufacturing cost. Many efforts have been made to improve the melting processability of the poly(vinyl alcohol), including process such as blending510 and chemical modification.1115 L.T. Sin et al.57 prepared the films of the polyvinyl alcohol (PVOH)/cassava starch (CSV) blends by a solution-cast method and investigated the synergistic interaction, thermal degradation, and activation energy of blend films using differential scanning calorimetry (DSC) and thermogravimetry (TGA). They8,9 also prepared the films of CSV filled with glycerol plasticized PVOH blends and investigated rheology, thermal behavior, and interactions of blend films. The results indicated that the PVOH blending with CSV had good improved thermal properties. J. Jang et al.10 studied the melting and crystallization behavior of polyvinyl alcohol (PVA) plasticized by glycerol. The results showed that the melting temperature Tm of PVA decreased with an increase in the amount of glycerin, and the effect of a plasticizer rapidly diminished when the phase separation of glycerin in PVA occurred. However, the blend films of PVA and other polymers were not thermoplastic and can only be prepared via solution casting. On the other hand, there are also few reports about graft polymerization of PVA with biodegradable and r 2011 American Chemical Society
hydrophobic aliphatic polyester, while these reactions between lactone or lactide and PVA must be performed under a rigorous water- and oxygen-free condition.1115 Moreover, since the PVA cannot undergo direct thermal processing, a large amount of monomer or organic solvent must be used for promoting the reaction. To address this issue, we have synthesized the PVA-g-LA copolymer by melt polycondensation in our previous work.16 It was found that the PVA-g-LA copolymer showed lower Tm and higher Td than PVA, which indicated that the thermal characteristic of the PVA-g-LA copolymer were obviously improved compared to that of pure PVA. As a result, it can be expected that the PLA side-chains may serve as an effective “internal” plasticizer to PVA, and endow PVA with an improved melt processing property. Therefore, the PVA-g-LA films can be prepared by melting compressed molding, which is unavailable for neat PVA. However, owing to high molecular weight and hydroxyl group content of PVA-1799, the strong inter- and intramolecular forces may benefit neither the complete mixing of PVA and LA nor the reaction activity between PVA and LA. Introduction of PVA0588, which has smaller molecular weight and lower OH contents than PVA-1799, is an effective method to resolve this problem. Therefore, the blend composed of PVA-0588 and PVA1799 with a certain ratio was used in the esterification of PVA with LA in this work. A complete knowledge of the material’s rheology would allow one to describe the extrusion behavior and also, by considering the appropriate boundary conditions, predict a material’s behavior during a given extrusion process. Therefore, it is important to investigate the rheological performance to determine the Received: February 25, 2011 Accepted: June 22, 2011 Revised: May 14, 2011 Published: June 22, 2011 9123
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Table 1. The Compositional Parameters of PVA-g-LA Copolymers Synthesized by Melt Polycondensation entry
weight fraction of PVA-0588 (%)
hydroxyl of PVA: LA (mol:mol)
DPa
DSa (%)
WPLAa (%)
PVA-g-LA-1
50
1:0.6
1.89
29.0
46.0
PVA-g-LA-2
50
1:0.5
1.78
26.2
42.1
PVA-g-LA-3
50
1:0.4
1.63
21.6
35.1
PVA-g-LA-4
50
1:0.3
1.64
13.2
25.4
PVA-g-LA-5
30
1:0.6
1.73
28.9
44.2
PVA-g-LA-6
20
1:0.6
1.79
28.0
44.4
PVA-g-LA-7
0
1:0.6
2.22
23.4
46.0
a
Calculated by 1H NMR as follows: DS is the number of grafted PLA chain; DP is the length of grafted PLA chain; WPLA stands for a PLA weight content in the graft product.
optimum condition for extrusion.17 A capillary rheometer is generally used to characterize the rheological properties of a wide variety of fluids. Capillary extrusion flow has been very often utilized for a wide variety of high-viscosity materials in an attempt to characterize their bulk intrinsic rheology as well as their wall interface boundary conditions. The technique has proved particularly advantageous in characterizing high-viscosity materials such as polymer melts and suspensions.18,19 For a variety of application fields and as the industrial importance of PVA increases, the determination of the capillary reological properties of PVA modified by lactic acid (PVA-g-LA) is necessary. The purpose of this work was to investigate (1) the rheological behavior of PVA-g-LA copolymers, such as the shear sensitivity, non-Newtonian index (n), the temperature sensitivity, and the viscous flow activation energy, (2) the influences of the copolymer’s structure on the rheological properties of PVAg-LA copolymers.
2. EXPERIMENTAL SECTION 2.1. Materials. In this experiment, two commercial grades of PVA were adapted and obtained from Chuanwei Company (Chongqing City, China) in granule form: PVA-1799 (degree of polymerization 1700, degree of hydrolysis 99%) and PVA0588 (degree of polymerization 500, degree of hydrolysis 88%). PVA is classified by the degree of hydrolysis and polymerization. The mainly used PVA is classified into the fully hydrolyzed grades (97.599.8% degree of hydrolysis) and the partially hydrolyzed grades (8789% hydrolysis).1,2 L-Lactic acid as an 88% aqueous solution was supplied by National Chemical Industry (Guangshui City, Hubei Province, China) and used without further treatment. Analytical grade stannous chloride (SnCl2.H2O) and acetone were purchased from Kelong Chemical Factory (Chengdu City, Sichuan Province, China), and used without further purification. 2.2. Preparation of PVA-g-LA Graft Copolymer. In our previous work,16 it is found that the esterification of PVA with lactic acid can provide (a) an improved melt processing ability and (b) an end product with improved thermal stability due to the introduction of PLA side-chains. Therefore, all the PVA-g-LA copolymer samples can form flowing melts at temperatures lower than their decomposition temperature, and can be measured by the capillary rheometer in the molten state. However, owing to high molecular weight and hydroxyl group content of PVA-1799, the strong inter- and intramolecular force may benefit neither the complete mixing of PVA and LA nor the reaction activity between the hydroxyl groups of PVA and LA. To resolve these problems, PVA-0588, which has lower molecular weight and
hydroxyl group content than PVA-1799, was introduced in this work. The mixture comprising PVA-0588 and PVA-1799 with a certain ratio showed much improved mixability with LA even at relative low LA content. The preparation procedure of all the PVA-g-LA copolymers followed the literature which was reported previously.16 In a typical example, 50.0 g PVA-1799, 50.0 g PVA-0588, 136.4 g 88% lactic acid solution in water, and 200 mL of distilled water were charged into a glass reaction kettle equipped with overhead stirrer. The reaction mixture was heated at 110 °C for about 1.5 h to obtain a clear melt mixture. Then the mixture was cooled to 80 °C and was moved into a vacuum kneader equipped with overhead stirrer. The reaction proceeded first for 2.5 h at 80 °C and low vacuum (0.070.08 MPa) to form a homogeneous system and to remove the bulk of water (the solvent). Thereafter 0.60 g SnCl2 3 H2O (0.5 wt % of lactic acid content) as catalyst was injected, and the reaction continued to proceed for another 3.5 h at 95 °C and high vacumm (0.1 MPa). The crude product was extracted with acetone for 48 h to remove the unreacted lactic acid and linear PLA oligomers. Finally, the purified product was dried at 60 °C under vacuum until constant weight was reached. 2.3. Characterization and Rheological Measurements. 1H NMR spectra (400 MHz) of PVA-g-LA copolymer samples were measured by using a Varian Inova 400 NMR apparatus. The measuring condition was as follows: solvent, d6-DMSO; solute concentration, 15% (w/v); internal standard, tetramethylsilane (TMS); temperature, 25 °C. The rheology measurements of PVA-g-LA copolymer samples were conducted using a high-pressure capillary rheometer (Rheograph 2003, Buchen, Germany) with a length-to-diameter ratio of 30 and an inlet angle of 90°. Measurements were carried out at 150, 155, 160, and 165 °C, respectively. The Rabinowitsch correction was applied to account for the influence of shear thinning in the calculation of the shear rate and corresponding viscosity, and the Bagley correction, corresponding to the adjustment for excess pressure drop at the die entrance, was applied by using three capillaries with the same radius but different length/ radius ratios. Non-Newtonian index characterizes the deviation from the Newtonian behavior. In the studied shear rate ranges, NonNewtonian behavior was described by the fitting of the experimental data (apparent viscosity-shear rate) with the power law equation: η ¼ Kγ 3 n 1
ð1Þ •
1
where K is the consistency coefficient, γ is the shear rate (s ) and n is the power-law index that is a non-Newtonian index. The 9124
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Figure 1. Relationship between apparent viscosity of PVA-g-LA with different structures and apparent shear rate at different temperatures.
plot of ln(η) against ln(γ•) for eq 1 should give a linear relationship, from which n can be determined from the slope.
respectively. DP ¼ ðI5:21 þ I4:08 Þ=I4:08
ð2Þ
3. RESULTS AND DISCUSSION
DS ¼ I4:08 =ðI5:13 þ I3:87 Þ
ð3Þ
3.1. Characterization of PVA-g-LA Copolymer. PVA was grafted with L-lactic acid by melt polycondensation using SnCl2 3 H2O as catalyst. PVA-g-LA copolymers with different molecular structures were characterized by 1H NMR.16 Among the signals typical for PVA bone of the PVA-g-LA copolymer in DMSO-d6, δ = 1.51.8 ppm (CH2CH), δ = 3.87 ppm (CH2CH(OH)) and δ = 4.16/4.44/4.70 ppm (OH), several new signals appear in the 1H NMR spectra: δ = 1.21.4 ppm (the grafted lactyls: CH3), δ = 5.21 ppm (the grafted lactyls: CH(CH3)O), δ = 4.08 ppm (the grafted lactyls: CH(CH3)OH) and δ = 5.13 ppm (CH2CHCOO), which is in great agreement with the esterified PVA,15,16,20 and confirms the grafting of LA onto PVA. Moreover, the 1 H NMR data were used for determining the molar substitution (MS) and the degree of substitution (DS), defined as an average number of introduced lactyl units and that of hydroxyls substituted for lactyl units, respectively, per hydroxyls residue of PVA, and the degree of polymerization (DP) meaning the length of PLA side-chain grafted. 15,16,20,21 In the present experiment, the values of DP, DS, and MS can be calculated by the following equations,
MS ¼ 1=ðDP DSÞ
ð4Þ
where I5.21 and I4.08 are resonance chemical shifts attributed to internal and terminal methine proton of lactyls, respectively; and I5.13 and I3.87 are resonance peak areas derived from methine groups with different chemical environments of PVA backbone. Accordingly, a PLA weight content (WPLA) in the graft product can be calculated with a molecular weight (47) of PVA repeating unit and that (72) of lactyl unit, in the following way:20 WPLA % ¼ 72MS=ð47 þ 72MSÞ100
ð5Þ
In the present experiment, the reaction conditions of the melt polycondensation of PVA with LA followed the literature which was previously reported.16 Furthermore, we carried out the changing manner of the compositional parameters of graft products with variation of in-feed molar ratio of PVA/LA or PVA0588 content. The compositional parameters of PVA-g-LA copolymers synthesized by melt polycondensation are summarized in Table 1. The content of PLA graft chains plays an important role in determining the properties of the copolymer. PVA-g-LA-14 9125
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Figure 2. Plots of apparent viscosity versus apparent shear rate of PVA-g-LA copolymers with different structures at 160 °C.
were conducted with different feed ratios of PVA to LA at fixed feed ratio of PVA-1799/-0588, reaction time, temperature, and dosage of catalyst (0.5 wt % of lactic acid content). The DS, DP, and WPLA increased with the increase of the PVA/LA ratio, suggesting that the increase of lactic acid content has benefited the copolymerization of LA grafted onto PVA. The similar results were reported for PLGA-grafted PVA.11,16 Owing to the lower molecular weight and less hydroxyl groups content of PVA-0588 compared to those of PVA-1799, incorporation a certain content of PVA-0588 with PVA-1799 may increase the molecular mobility and decrease the inter- and intramolecular forces and therefore promote not only the complete mixing of PVA and LA but also the reaction activity between PVA and LA. The influence of PVA-0588 content on the PVA-g-LA products was also studied as shown for PVA-g-LA-1 and 57, which were carried out with the same feed ratio of PVA/LA, reaction time, temperature, and dosage of catalyst (0.5 wt % of lactic acid content). 3.2. Rheological Properties of PVA-g-LA Copolymers. Poly(vinyl alcohol) (PVA) is a semicrystalline polymer containing hydroxyl groups which generate inter- and intramolecular hydrogen bonding. Hydrogen bonding has a profound effect on the rheological and mechanical properties of the polymer. The extent of hydrogen bonding is greatly affected by stereoregularity of hydroxyl groups,22 depending on the density and spatial arrangement of hydroxyl groups. The molecules of neat PVA were arranged in an organized pattern, which provided strong inter- and intramolecular forces and great resistance to flow. Thus, neat PVA cannot be measured by capillary rheometer in the molten state. 3.2.1. Rheological Behavior. Figure 1 shows the flow curves of PVA-g-LA copolymers with different structures, as apparent viscosity (pa 3 s) versus apparent shear rate (s1). In the studied shear rate ranges, all the PVA-g-LA copolymer samples with different structures belong to pseudoplastic or shear thinning fluids. It is evident from the figure that the apparent viscosity of all the PVA-g-LA copolymers decreased linearly with increasing shear rate.17 Pseudoplastic or shear-thinning fluids have a lower shear viscosity at higher shear rates. It is generally supposed that the large molecular chains tumble at random and affect large
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volumes of fluid under low shear but that they gradually align themselves in the direction of increasing shear and produce less resistance.2325 3.2.2. Effect of the Copolymer’s Structure. Figure 2 presents the plots of apparent viscosity versus apparent shear rate of PVA-g-LA copolymers with different structures at 160 °C. The apparent viscosities of samples PVA-g-LA-1, 2, 3, and 4 have been reduced by having a higher amount of PLA grafting chain. More PLA grafting chain content may lead to lower interaction between PVA-g-LA copolymer chains, and promote movement among the copolymer molecule.26 In spite of that, the apparent viscosities of samples PVA-g-LA-1, 5, 6, and 7 decreased with increasing PVA-0588 content at the constant shear rate. The PVA-0588 has much smaller molecular weight than PVA-1799, which may increase the chain mobility of PVA-g-LA copolymers and decrease the entanglements between molecular chains leading to a decrease in viscous resistance to flow. Thus the PVA-g-LA copolymer incorporated with higher PVA-0588 content showed lower apparent viscosity. The much improved flowability of the PVA-g-LA copolymer in the molten state imbued it with a good thermoplastic processability, which is unavailable for neat PVA.26 As shown in Figures 1 and 2, the shear viscosity of PVA-g-LA copolymer samples diminished with an increase in shear rate. Although all samples showed similar shear thinning behaviors, the extent of shear thinning varied with the molecular structures of the copolymers, suggesting the dependence of flow behavior on the molecular structures of PVA-g-LA copolymers.26 These variations can be evaluated and described by the calculation of the shear sensitivity index (I(Y )T) and non-Newtonian index (n). The shear sensitivity index (I(Y )T) characterizes the shear rate sensitivity of apparent viscosity. The shear rate can be adjusted to achieve changes in viscosity effectively as the stronger shear dependence of viscosity. Therefore, this index (I(Y )T) can be a measure of the dependence of shear rate on viscosity. At a constant temperature, the shear sensitivity index can be described by the following equation:27 IðY ÞT ¼ ½ηðY 1 Þ=ηðY 2 ÞT
ð6Þ
where η(Y 1) is the apparent viscosity of Y 1 =100 s1 at constant temperature, η(Y 2) is the apparent viscosity of Y 2 = 1000 s1 at constant temperature, and I(Y )T is the shear sensitivity index. According to the flow curves of PVA-g-LA copolymers (as shown in Figure 2), the shear sensitivity indexes (I(Y )T) of the copolymers with different structures at 160 °C were obtained and listed in Table 2a. On the whole, the I(Y )T values of PVA-g-LA copolymers increased from 4 to 6 with a higher PLA grafting chain content or the PVA-0588 content, which means that the flow behavior of PVA-g-LA copolymer was sensitive to shear rate with a higher PLA grafting chain content or the PVA-0588 content.26 In other words, the I(Y )T values of PVA-g-LA copolymers increased with increasing PLA grafting chain content or the PVA-0588 content. The results were in great agreement with the previous ones that were shown in Figure 2. These phenomena were explained that with increasing PLA grafting chain content or PVA-0588 content, the molecular chains of PVA-g-LA copolymers were better oriented and the number of entanglements was reduced, which results in decreased interactions between chain segments by shearing.28 According to the flow curves of PVA-g-LA copolymers (as shown in Figure 1), Table 2b gives the shear sensitivity index 9126
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Table 2 (a) Shear Sensitivity Index (I(Y )T) of PVA-g-LA Copolymers with Different Structures at 160 °C η (Pa 3 s) sample
a
WPLA (%)
DP
DS (%)
Y 1
Y 2
I(Y )T
PVA-g-LA-1
46.0
1.89
29.0
1022.2
187.6
5.45
PVA-g-LA-2
42.1
1.78
26.2
2251.6
391.6
5.75
PVA-g-LA-3
35.1
1.63
21.6
2576.4
505.1
5.10
PVA-g-LA-4
25.4
1.64
13.2
3281.7
740.7
4.43
PVA-g-LA-5 PVA-g-LA-6
44.2 44.4
1.73 1.79
28.9 28.0
1282.0 776.4
248.3 192.9
5.16 4.02
PVA-g-LA-7
46.0
2.22
23.4
456.9
109.4
4.18
a
a
b
)T) of PVA-g-LA Copolymers at Different Temperatures (b) Shear Senstitivity Index (I(Y η (Pa 3 s) temperature (°C)
Y 1
PVA-g-LA-1 (WPLAa = 46.0%; DPa = 1.89; DSa = 29.0%.)
150
1284.8
287
4.48
155 165
1226.0 718.1
268.4 187.4
4.57 3.83
155
2342.8
396.7
5.91
160
2251.6
391.6
5.75
165
1270.9
268.1
4.74
150
2050.7
357.0
5.74
160 165
1282.0 983.8
248.3 206.9
5.16 4.75
PVA-g-LA-2 (WPLAa = 42.1%; DPa = 1.78; DSa = 26.2%.)
PVA-g-LA-5 (WPLAa = 44.2%; DPa = 1.73; DSa = 28.9%.)
a
I(Y )Tb
sample
Determined by 1H NMR. b I(Y )T = [η(Y 1)/η(Y 2) ]T.
Y 2
Table 3 (a) Non-Newtonian Index n of PVA-g-LA Copolymers with Different Structures at 160 °C WPLAa(%) DPa DSa (%) n
sample
R2
PVA-g-LA-1
46.0
1.89
29.0
0.26
0.9955
PVA-g-LA-2
42.1
1.78
26.2
0.24
0.9972
PVA-g-LA-3
35.1
1.63
21.6
0.29
0.9971
PVA-g-LA-4
25.4
1.64
13.2
0.35
0.9979
PVA-g-LA-5
44.2
1.73
28.9
0.29
0.9996
PVA-g-LA-6
44.4
1.79
28.0
0.40
0.9872
PVA-g-LA-7
46.0
2.22
23.4
0.38
0.9902
(b) Non-Newtonian Index n of PVA-g-LA Copolymers at Different Temperatures n at different temperature
a
sample
WPLAa (%)
DPa
DSa (%)
150 °C
155 °C
PVA-g-LA-1
46.0
1.89
29.0
0.349
0.340
PVA-g-LA-2
42.1
1.78
26.2
PVA-g-LA-5
44.2
1.73
28.9
0.241
PVA-g-LA-7
46.0
2.22
23.4
0.305
0.229 0.366
160 °C
165 °C 0.417
0.240
0.324
0.287
0.303
0.379
Determined by 1H NMR.
of PVA-g-LA copolymers at different temperatures. It is clear that the shear sensitivity index decreased with increasing the temperature for each of PVA-g-LA samples, implying that the effect of the shear rate on the apparent viscosities of PVA-g-LA
copolymer samples became less pronounced at higher temperature. The above results indicate that both the variations in PLA grafting chain content and PVA-0588 content affect the rheological 9127
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Figure 3. Relationships between apparent viscosity and temperature for PVA-g-LA copolymers with different structures at different shear rates (s1): 1, 50; 0, 100; g, 200; 3, 500; 4, 800; 9, 1000.
behavior of PVA-g-LA copolymer melts. This means that the rheological behavior of PVA-g-LA copolymers with different structures has different responses to the changes of experimental conditions including temperature and shear rate. 3.2.3. Non-Newtonian Index (n) for PVA-g-LA Copolymer. The characterization for rheological properties of PVA-g-LA copolymers with different structures can be obtained by calculating and comparing the rheological parameters such as nonNewtonian index and viscous flow activation energy. The nonNewtonian index (n) is a term from the power law equation (η = Kγn-1), that is ln(η) = ln(K) + (n 1) ln(γ). Therefore, the slopes of the line in Figure 2 can be used for calculating the nonNewtonian index (n). The values of n different from unity describe the deviation from Newtonian fluids. The fact that n values were less than unity indicates that these products are pseudoplastic materials.17 A lower n value indicates a greater degree of deviation from Newtonian fluids and a higher influence of shear rate on viscosity.26 Table 3a shows the non-Newtonian index (n) values of PVAg-LA copolymers with different structures at 160 °C, from which all the PVA-g-LA copolymer melts exhibited shear-thinning (i.e., pseudoplastic) behavior because the values of non-Newtonian index (n) were less than 1. Besides, in all cases, the coefficients of determination (R2) were higher than 0.98. The R2 values
obtained (Table 3a) were extremely close to 1, which confirmed the Power Law model to be adequately suitable for describing the flow behavior of the PVA-g-LA copolymers within the range studied.17 Moreover, the molecular structure of PVA-g-LA copolymer may influence the n value. As shown in Table 3a, the n values of PVA-g-LA samples decreased with increasing of PLA grafting chain content, while decreasing with an increase of PVA-0588 content at similar PLA grafting chain content. This is mainly due to a significant decrease in frictional resistance and an increase in sliding between molecular chains with the increase of PLA grafting chain content or higher PVA-0588 content.26 These data show that both higher PLA grafting chain content and higher PVA-0588 content lower the shear dependence of viscosity for the PVA-g-LA copolymer. Therefore, it is preferable to increase the flowability of PVA-g-LA copolymers by adjusting the shear rate.29 Table 3b gives the non-Newtonian index (n) of PVA-g-LA copolymers at different temperatures as well. For the same sample, the flow index (n) increased (Table 3b) with the test temperature, suggesting that the melts became less pseudoplastic (higher n values) at higher temperature.28 The main reason for this is that the ability of molecular chain motion of the PVA-g-LA copolymer is enhanced and the resistance between 9128
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Figure 4. Relationships between viscous flow activation energy and shear rate for PVA-g-LA copolymers with different structures.
the copolymer melt layers decreases by increasing the processing temperature, leading to a reduction in the shear dependence of viscosity.28 Moreover, at a relatively higher temperature, there is more energy available for the destruction of entanglements. 3.2.4. Viscosity Activation Energy for PVA-g-LA Copolymer. The effect of temperature on the rheological properties of PVA-g-LA copolymers with different structures is illustrated in Figure 1, from which it can be seen that the apparent viscosities of PVA-g-LA copolymers were lower at higher temperature than those values obtained at lower temperatures. The viscosity of a fluid is affected by the binding between molecules which is affected by temperature. When external energy is supplied by heating to increase temperature, it increases the energy of the molecules. The decrease in viscosity can be attributed to the increase in intermolecular distances, because of the thermal expansion caused by the increase in temperature.17 The temperature effect on the apparent viscosity of PVA-g-LA copolymers at a constant shear rate may be modeled by an Arrhenius-type equation, which could be expressed by ln η ¼ ln A þ Eη =RT
ð7Þ
where T is the temperature in Kelvin, A is a material constant, Eη is the viscous flow activation energy, and R is the universal gas constant. The parameters of the Arrhenius equation describing the viscositytemperature relationship were determined by linear regression. The Arrhenius model gave a good description of the temperature effect on apparent viscosity at different constant shear rates as shown in Figure 3 and Figure 4. From Figure 3, it was observed that there was a linear relationship of ln(η) versus (1/T). The experimental data showed that the apparent viscosities of PVA-g-LA copolymers with temperature obey the Arrheniustype equation.17 Figure 4 shows the viscous flow activation energies of PVA-g-LA copolymers obtained from linear regression on semilogarithmic plots of apparent viscosity versus temperature. From Figure 4, it was found that the Eη value of the PVA-g-LA copolymer melts decreased sharply with increasing shear rate, implying that the effect of the test temperature on the melt viscosity became less pronounced at higher shear rate. An increase in temperature led to an increase in the flowability for
polymer segments.28 Accordingly, the viscosity of polymer melts decreased. For most polymers, the resistance becomes lower at higher shear rate, because the disentanglement of the molecular chains is stronger than the entanglement, which leads to a reduced Eη.29 Usually, the viscous flow activation energy (Eη) represents the temperature sensitivity of the apparent viscosity. Larger Eη means a higher dependence of temperature on viscosity.29 According to Steffe,30 in a system, higher Eη values indicate a more rapid change in viscosity with temperature. The Eη values varied with the molecular structures of PVA-g-LA copolymers. This reveals that there is diversity in temperature sensitivity. As seen in Figure 4, the Eη values of sample PVA-g-LA-1 and 2 decreased with increase of the PLA grafting chain content and the Eη values of sample PVA-gLA-1, 5, and 7 decreased with increase of PVA-0588 content at a constant shear rate. Activation energy (Eη) is necessary for movement of a molecule, and the viscous flow activity energy is determined by the transition resistance of chain segments. As PLA grafting chain content or PVA-0588 content of the PVA-gLA copolymers increased, the transition resistance of chain segments of the copolymers was reduced and the melts flowed more easily.30 Thus, adjusting the molecular structure was a very effective way of regulating the flowability of PVA-g-LA copolymers.
4. CONCLUSIONS The rheological properties of PVA-g-LA with different molecular structures were studied using capillary rheometer. The PVA-g-LA samples exhibited noticeable shear thinning behavior and the apparent viscosities of PVA-g-LA were sensitive both to temperature and to shear rate. Both the variations in PLA grafting chain content and PVA-0588 content have obvious effects on rheological properties of PVA-g-LA copolymers. The apparent viscosity of PVA-g-LA decreased with the increase of PLA grafting chain content or PVA-0588 content at a constant temperature and shear rate. Both the PLA grafting chain content and the PVA-0588 content of PVA-g-LA copolymers lower the melt viscosity and led to an increase in non-Newtonian index and viscous flow activation energy. Moreover, PVA-g-LA copolymer melts with PLA grafting chain content and PVA-0588 content both lead to a decrease in shear dependence and an increase in temperature sensitivity of viscosity, in consequence of the lowering in molecular interaction and entanglement concentration. Therefore, introduction of the oligo-poly (lactic acid) graft chain or the incorporation of PVA-0588 can give rise to the melting flow processing of PVA-g-LA copolymers, and the flowability could be regulated by adjusting the molecular structure. ’ AUTHOR INFORMATION Corresponding Author
*Tel. and fax: +86-28-85410259. E-mail:
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’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 20874065). ’ REFERENCES (1) Finch, C. A. Polyvinyl Alcohol; Wiley: New York, 1992; pp 13; 1218. 9129
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