Preparation and Characterization of High-Melt-Strength Polylactide

Article pubs.acs.org/IECR

Preparation and Characterization of High-Melt-Strength Polylactide with Long-Chain Branched Structure through γ‑Radiation-Induced Chemical Reactions Hongjun Xu, Huagao Fang,* Jing Bai, Yaqiong Zhang, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China ABSTRACT: An easy procedure was applied to prepare high-melt-strength polylactide (PLA) that involves γ-radiation-induced free-radical reactions to introduce a long-chain branched structure onto a linear PLA precursor with addition of a trifunctional monomer, trimethylolpropane triacrylate (TMPTA). The results from size-exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) detection indicate that the resultant long-chain branched PLA (LCB PLA) samples have an increased molecular mass and an elevated branching degree with increasing amount of TMPTA incorporated during the irradiation process. Various rheological plots including viscosity, storage modulus, loss tangent, Cole−Cole plots, and weighted relaxation spectra were used to distinguish the improved melt strength for LCB PLA samples. The effect of LCB structure on elongational rheological properties was further investigated. The LCB PLA samples exhibited an enhancement of strainhardening under elongational flow. The enhanced melt strength substantially improved the foaming performance of the LCB PLA samples.

1. INTRODUCTION Polylactide (PLA) is a type of aliphatic thermoplastic polyester that is usually prepared by ring-opening polymerization of lactide, a dimer of lactic acid. PLA exhibits a number of interesting properties including biodegradability, biocompatibility, sufficient mechanical properties, and processability. Therefore, it has received great attention in recent years because of its potential to replace conventional petrochemicalbased polymers.1−4 However, the commercially available PLA with a linear chain structure has limited applications in several processes in which high melt strength and strong extensional properties are required, such as thermoforming, foaming, blow molding, and fiber spinning.5−8 Several methods have been applied to improve the melt strength of PLA, which is accomplished by incorporating some new functional groups into the backbone chains of PLA, blending with other polymers, or adding some fillers.9−13 On the basis of the research on conventional polymers such as polyethylene and polypropylene, the melt rheological properties can also be improved by modifying the chain topology, that is, introducing a long-chain branched (LCB) structure into the polymer materials.14−22 A long-chain branched structure means that the molecular chain between two branch points in the melt is long enough to entangle with other chains, and the enhanced entanglements result in a high melt strength. As compared with linear polymers of equal molecular mass, LCB polymers exhibit unique rheological properties including higher Newtonian viscosity, more pronounced shear thinning, stronger melt elasticity and higher storage modulus, and enhanced strain hardening under elongational flow.14−18 Several methods have been developed to obtain PLA with long-chain branched structures. One common method is to apply the ring-opening polymerization of lactide or the polycondensation of lactic acid with the introduction of © 2014 American Chemical Society

multifunctional co-initiators or monomers in the solutions. From the standpoints of environmental protection, cost, effectiveness, and processing convenience, solution reactions are not the best choice.23−26 Another method is to carry out the branching reactions by reactive processing in the melt in the presence of some free-radical initiators or multifunctional chain extenders, which is evidently more convenient and cheaper and can produce a large volume of polymers.27−33 Multifunctional monomers such as epoxy,29,30 anhydride,31 diisocyanate,32 and acrylate33 have been used to prepare LCB PLA samples with higher molecular masses and improved melt strengths as compared with their linear precursors. The newly developed method for introducing branches is high-energy irradiation, which induces free-radical reactions of a linear PLA precursor with a multifunctional monomer.34,35 Electron beams and γrays are two types of generally utilized high-energy radiation resources for polymer topological modification. Notwithstanding the widespread applications of high-energy irradiation in polymer modifications, there are some emerging new understandings regarding high-energy-irradiation-initiated branching reactions. Auhl et al. recently reported different effects of highenergy irradiation in the fabrication of long-chain branched polypropylene (LCB-PP), for which γ-irradiation on PP leads to some higher-molecular-mass component, whereas electron beam irradiation does not show such an effect.36 In our previous publications, we reported that LCB PLA samples modified by γ-irradiation exhibited a bimodal architecture with both a short linear chain fraction and a tree-like LCB fraction, as compared with samples obtained by electron beam Received: Revised: Accepted: Published: 1150

October 30, 2013 December 13, 2013 December 27, 2013 January 7, 2014 dx.doi.org/10.1021/ie403669a | Ind. Eng. Chem. Res. 2014, 53, 1150−1159

Industrial & Engineering Chemistry Research

Article

irradiation, which showed only the monomodal macromolecular structure.35,37 In this work, the modification of the chain topology of PLA by applying γ-radiation with and without addition of the multifunctional monomer TMPTA was investigated. The linear viscoelastic properties and uniaxial elongational flow behaviors of these LCB PLA samples were further characterized. Finally, the improved melt strength was demonstrated by the improved foaming performance of LCB PLA samples.

Table 1. Experimental Conditions for the Preparation of PLA Samples and Their Gel Contents

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial polylactide (PLA) (Natureworks product PLA2002D with 96 wt % L-isomeric content) was used in this study as a linear PLA precursor. The mass density and melt flow index for PLA2002D were 1.24 g/cm3 and 4.8 g/10 min (190 °C, 2.16 kg), respectively. The multifunctional monomer trimethylolpropane triacrylate (TMPTA, 99% purity) was obtained from Tianjin Tianjiao Chemical Company, Tianjin, China. Acetone and chloroform were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Tris(nonylphenyl) phosphate (TNPP) was purchased from the Sigma-Aldrich, St. Louis, MO. All chemicals were used as received. 2.2. Sample Preparation. A series of linear PLA samples were prepared by γ-irradiation under vacuum conditions. We note here that the γ-irradiation process in ambient atmosphere leads to more obvious molecular mass loss for PLA samples as compared with samples irradiated in a vacuum at the same irradiation doses. The PLA samples become yellowish under irradiation in ambient atmosphere, and the degree of yellowing increases with increasing irradiation dose. For practical applications, we suggest that the irradiation process be carried out under the protection of an inert gas such as N2 or Ar. To prepare the long-chain branched PLA samples, stoichiometric amounts of TMPTA were first melt-mixed with 50 g of linear PLA precursor in an XSS-300 torque rheometer. Linear PLA precursor was subjected to the same treatment to ensure the same thermal history. The obtained granular samples (mixed with TMPTA) were sealed into glass tubes under a vacuum for subsequent irradiation by γ-rays of 2.22 kGy/h from a 60Co source. After being irradiated at different doses, the samples were annealed at 80 °C for 30 min, allowing sufficient migrations of chain fragments for the reaction. Then, the residual free radicals were deactivated by annealing the samples at 100 °C for 30 min. In this work, sample codes PLA0, PLA1, PLA2, and PLA3 represent the samples irradiated at 0, 5, 10, and 15 kGy, respectively, and sample codes LCB-PLA1, LCBPLA2, LCB-PLA3, and LCB-PLA4 represent the samples containing 0.2, 0.4, 0.6, and 0.8 wt % TMPTA, respectively, that were irradiated at 5 kGy. Table 1 summarizes the formulation conditions for sample preparation. Special care was taken in handling all of the samples to exclude moisture to decrease the possibility of hydrolytic degradation. 2.3. Determination of Gel Content. The irradiated PLA samples were dissolved in a 20-fold excess of chloroform for 24 h. The soluble portions were precipitated in cold methanol and then dried in a vacuum to constant mass to be ready for rheological measurements. The gel contents were estimated as the ratios of the masses of the dried gels (residual undissolved portions) to the initial masses of the irradiated PLA samples. 2.4. Characterization of Molecular Masses and Molecular Mass Distributions. The characterization of the PLA samples with respect to the molecular mass distribution

sample code

TMPTA content (wt %)

radiation dose (kGy)

gel content (wt %)

PLA0 PLA1 PLA2 PLA3 LCB-PLA1 LCB-PLA2 LCB-PLA3 LCB-PLA4

0 0 0 0 0.2 0.4 0.6 0.8

0 5 10 15 5 5 5 5

0 0 0 0 0 0 0.15 6.7

and long-chain branching level was carried out using a sizeexclusion chromatograph coupled with a DAWN HELEOS II multiangle laser light scattering detector, an Optilab T-rEX refractive index (RI) detector, and a ViscoStar II viscometer. The measurements were performed at 30 °C. The eluent, tetrahydrofuran (THF), was used at a flow rate of 1.0 mL/min. The molecular mass parameters were calculated from the sizeexclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) data using the commercial software ASTRA 6 (Wyatt Technology, Santa Barbara, CA). The obtained values of molecular mass and molecular mass distribution for the PLA samples are listed in Table 2. Note that values for LCB-PLA4 could not be obtained because of the high viscosity of the solution. 2.5. Measurements of Rheological Properties. Disk-like PLA samples with a thickness of about 0.9 mm and a diameter of 25 mm for rheological measurements were prepared by mold-compression at 180 °C under a vacuum in a hot press. TNPP was added at an optimum concentration of 0.35 wt % to stabilize the PLA samples during rheological measurements, in accordance with the reports of Palade et al.5 and Othman et al.38 Dynamic rheological measurements were performed on a TA-AR2000EX rotational rheometer (TA Instruments, New Castle, DE) in a parallel-plate geometry with a diameter of 25 mm and a gap of 0.9 mm at 180 °C in nitrogen atmosphere. The thermal stability was ensured by time sweep for all samples under the testing conditions. Frequency sweeps were carried out at a fixed strain of 2% to measure the linear viscoelastic properties for linear PLA and LCB PLA samples. Uniaxial elongational flow measurements were carried out on an ARES G2 rheometer (TA Instruments, New Castle, DE) at 170 °C with an extensional viscosity fixture (EVF) at constant strain rates of 0.05, 0.1, 0.3, and 0.5 s−1. The sample sheets were cut into pieces with a width of 10 mm and a length of 17 mm for the measurements. A prestretch for 9 s was performed before each measurement to ensure that no slipping occurred between the sample and the fixture. 2.6. Batch Foaming by Carbon Dioxide and Morphological Observation. The foaming behaviors of linear PLA precursor and LCB PLA samples were investigated by batch foaming process using compressed carbon dioxide (CO2) as the foaming agent. Specimens with a thickness of 0.5 mm were prepared by compression molding under a vacuum at 180 °C and were then cut to dimensions of 10 mm × 10 mm × 0.5 mm for foaming. Following saturation with CO2 at 5 MPa for 12 h, the specimens were depressurized rapidly and transferred into a water bath at predetermined temperatures within 1 min for foaming. The foamed specimens were fractured in liquid nitrogen and then sputter-coated with gold for morphological 1151

dx.doi.org/10.1021/ie403669a | Ind. Eng. Chem. Res. 2014, 53, 1150−1159


Article

Table 2. Molecular Parameters Determined by SEC-MALLS and Rheological Parameters for Linear PLA and LCB PLA Samples sample code

Mwa (kg/mol)

Mw/Mna

|η*| at 0.05 rad/sb (Pa·s)

terminal slope of G′

τLc (s)

τBc (s)

PLA0 LCB-PLA1 LCB-PLA2 LCB-PLA3 LCB-PLA4

112 114 169 269 −

1.4 1.7 2.2 3.3 −

4007 4250 9400 11550 32200

1.8 1.5 1.4 1.3 0.8

0.03 0.03 0.03 0.03 −

− 0.3 2.0 2.8 −

a c

Determined by SEC-MALLS. Note that the values for LCB-PLA4 are outside the measurement limitation. bValue for PLA0 obtained at 0.1 rad/s. Values obtained from weighted relaxation spectra.

Figure 1. (a) Changes in complex viscosity, |η*(ω)|, as a function of angular frequency, ω, and (b) changes in phase angle, δ, as a function of complex modulus, |G*|, for PLA irradiated at various doses and for the nonirradiated PLA sample (PLA0).

and the δ values are nearly 90° at the low value end of |G*|. Any deviations from the universal curve can be referred to the influences of long-chain branches or broadening of the molecular mass distribution. The only obvious difference is an extension toward higher loss angle and lower modulus values with increasing irradiation dose. From the fact that these curves are mere extensions of each other, without any changes in shape, we conclude that there are no significant changes in the molecular mass distribution and linear structure of PLA with increasing irradiation dose. These results are consistent with those in the literature.41 Nugroho et al. reported that PLA is predominantly degraded by random chain scission under γirradiation up to 200 kGy rather than by the introduction of branching or cross-linking structures, which can be ascribed to its unique chain structure.41 The degradation of PLA under γ-irradiation can be restrained by mediating the free-radical reactions using the multifunctional monomer TMPTA. It is well documented in the literature that free macroradicals can be induced by γ-irradiation in PLA, as studied using electron paramagnetic resonance (EPR) spectroscopy.39,40 The macroradicals tend to be terminated if no active functional groups exist in the excited PLA matrix, resulting only in a decrease of the molecular mass. With addition of the multifunctional monomer TMPTA, the macroradicals can attach to one CC group on TMPTA to realize the first grafting reaction, and the same reaction can happen to the other CC groups by other macroradicals that form during the irradiation process. Finally, long-chain branches or cross-linking structures can form in PLA, depending on the amount of TMPTA incorporated. The gel contents listed in Table 1 indicate that the fractions of crosslinking structures in the irradiated samples were tiny for low incorporated amounts of TMPTA.

observation by field-emission scanning electron microscopy (SEM; FEI Sirion 200).

3. RESULTS AND DISCUSSION 3.1. Effect of γ-Irradiation on PLA Topological Modification. The exposure of polymers to high-energy γrays can cause various irradiation-induced reactions according to the free-radical initiation mechanism, including degradation, cross-linking, branching, and/or grafting. PLA is sensitive to γirradiation.39−41 In this work, the effects of γ-irradiation on the changes in molar mass and molecular structure of linear PLA precursor without addition of TMPTA were first investigated by analyzing its linear rheological properties. Figure 1a shows the changes in complex viscosity, |η*|, as a function of angular frequency, ω, for PLA samples irradiated at various doses and for the nonirradiated PLA sample (PLA0). The complex viscosities of irradiated PLAs decreased over the entire frequency range with increasing irradiation dose. At low frequencies, the zero-shear viscosity reached a value of 4100 Pa·s for nonirradiated neat PLA and a lower value of 420 Pa·s for the sample irradiated at a dose of 15 kGy, indicating that the decrease of molecular mass occurred because of chain scission during the irradiation process. Changes in the topological structure of polymers can be reflected in their linear viscoelastic properties. The van Gurp− Palmen plot is a useful tool for identifying the relaxation species in a complex polymeric system. This plot (phase angle, δ, versus absolute complex modulus, |G*|) not only serves to evaluate polydispersity effects on the linear viscoelastic response of linear polymers but also provides information on changes in the topological structures of polymers.42−44 Figure 1b shows a van Gurp−Palmen plot for PLA irradiated at various doses, as well as the nonirradiated PLA sample (PLA0). All of the δ−|G*| curves overlap to form one “universal” curve, 1152



Article

Figure 2. (a) SEC curves obtained with a refractive index detector and (b) Mark−Houwink−Sakurada plots for linear PLA precursor and LCB PLA samples.

viscosities, |η*(ω)|, of the linear PLA sample (PLA0) and the LCB PLA samples are plotted as a function of the applied frequency in Figure 3a. The complex viscosity of the LCB PLA samples increased with increasing amount of TMPTA incorporated in the reactions at low frequencies (see Table 2), and the increase in complex viscosity disappeared at the high frequencies. Figure 3a also displays two types of frequency dependence of the complex viscosity. For linear PLA0, the complex viscosity remained constant over a certain range in the

The effects of TMPTA content on the topological modification of PLA were studied using SEC coupled with multiangle laser light scattering (SEC-MALLS). The obtained molecular parameters, such as the weight-average molecular mass, Mw, and the polydispersity index, Mw/Mn, are listed in Table 2. It was found that, although the incorporated TMPTA could not be distinguished by 1H NMR spectroscopy (data not shown), the molecular mass and chain structures were modified a great deal by γ-irradiation-induced reactions of PLA with TMPTA. Figure 2a shows the SEC curves determined with the refractive index (RI) detector for linear PLA precursor and LCB PLAs. The linear PLA precursor (PLA0) shows only one peak in its RI trace. In contrast, the samples containing TMPTA (LCB PLA1−3) exhibit a shoulder or peak at short retention times, and the relative intensity of the shoulder or peak increases with increasing TMPTA content, indicating that more PLA chains with higher molar masses are generated upon addition of more TMPTA for the irradiation process. SEC-MALLS measurements not only provide the absolute molecular mass, but also reveal information about the branching structures by investigating the dependences of the radius of gyration, Rg, and intrinsic viscosity, [η], on the molecular mass, MLS. Generally, a branched macromolecule has lower Rg and [η] values compared with a linear macromolecule of the same molecular mass.45 The double-logarithmic plot of [η] as a function of molecular mass for the PLA samples, generally referred to as the Mark−Houwink−Sakurada plot, can be used to qualitatively determine the LCB structure. As shown in Figure 2b, the curve for the linear PLA precursor is a straight line with a slope of 0.75. For LCB PLA samples, the intrinsic viscosity, [η], deviates from the linear reference to lower values for the higher molecular mass fraction, indicating contraction of the coiling chains in the solutions. The degree of deviation becomes more pronounced as the TMPTA content increases, implying an increasing long-chain branching level in the LCB PLA samples. 3.2. Linear Viscoelastic Properties of LCB PLA Samples. The melt strength of polymers is often reported as the elastic behavior under shear flow. In this study, smallamplitude oscillatory shear measurements were performed at a temperature of 180 °C to examine the effect of added TMPTA on the improvement of the melt strength, such as the complex viscosity, storage modulus, loss modulus, and loss tangent, for LCB PLA samples. The presence of a low amount of LCB can change the viscosity and degree of shear thinning as compared with those of linear polymers with similar molecular masses. The complex

Figure 3. (a) Changes in complex viscosity, |η*|, as a function of angular frequency, ω, and (b) Cole−Cole plots for linear PLA precursor and LCB PLA samples at 180 °C. Inset: Cole−Cole plots at a lower viscosity scale. 1153



Article

low-frequency region and then decreased with further increasing frequency, indicating a transition from the Newtonian plateau to the power-law regime at the inflection point. For LCB PLA samples, the Newtonian behavior was hard to observe in the experimental frequency range, as illustrated by the absence of a frequency-independent viscosity, indicating that the LCB PLA samples had a more pronounced power-law region at lower frequencies. The enhancement of the shear thinning behavior for the LCB PLA samples became stronger with increasing amount of TMPTA incorporated. The nonterminal behavior of LCB PLA samples can also be illustrated by a Cole−Cole plot (η″−η′ plot) as shown in Figure 3b. The Cole−Cole plot for linear PLA0 is arc-shaped. The LCB PLA samples exhibited different curves, for which the radius of the arc became larger than that of linear PLA0 and showed more pronounced upturning at high viscosities with increasing amount of TMPTA incorporated. Eventually, sample LCB-PLA4 displayed monotonically increasing values of η″ with increasing η′ because of its elastic solid-like feature caused by the high degree of long-chain branching. In addition to the complex viscosity, the change in storage modulus, G′, with frequency is also sensitive to the LCB level. The changes in storage modulus, G′, and loss modulus, G″, as functions of angular frequency, ω, for linear PLA0 and LCB PLA samples are shown in panels a and b, respectively, of Figure 4. It was found that all of the samples had almost identical G′ and G″ values at high frequencies. With decreasing measured frequency, both G′ and G″ decreased, with PLA0 falling most rapidly and LCB-PLA4 falling most slowly. Moreover, the G′ and G″ values of the samples at low frequencies followed the amounts of TMPTA incorporated in the γ-irradiation-induced reactions. In the terminal region, the storage modulus, G′, and loss modulus, G″, for linear polymers follow the well-known frequency dependences, that is, G′ ∝ ω2 and G″ ∝ ω, for which only the longest relaxation times contribute to the viscoelastic behavior. The terminal slopes for linear PLA0 and LCB PLA samples are listed in Table 2. The deviation from an ideal fluid at low frequencies suggests higher elasticity for LCB PLA samples as compared with linear PLA0, which can be attributed to more entanglements because of the existence of long-chain branches. The higher elasticity of the LCB PLA samples compared with linear PLA0 can also be illustrated by the frequency dependences of the loss tangent (tan δ = G″/G′) as shown in Figure 4c. For linear PLA0, the curve rises with decreasing frequency, which is a typical terminal behavior for a liquid-like material. Behaving differently, the LCB PLA samples demonstrated a gel-like behavior, showing much less significant frequency dependences of the loss tangent. With the addition of TMPTA, the loss tangent of the LCB PLA samples decreased quickly at low frequencies. The loss tangent decreased continually, and a plateau was eventually reached when more TMPTA was incorporated, indicating the increased LCB level with the greater incorporation of TMPTA in γirradiation. The gel-like behavior in these LCB PLA samples is consistent with that observed for other LCB polymer materials.46,47 To investigate the effects of long-chain branched structure on the relaxation behaviors of linear PLA0 and LCB PLA samples, the weighted relaxation spectra were obtained from the G′ and G″ data using the standard nonlinear regularization method in the Trios Software (TA Instruments, New Castle, DE), and the results are shown in Figure 5. It can be clearly seen that the

Figure 4. Changes in (a) storage modulus, G′, and (b) loss modulus, G″, as functions of angular frequency, ω, and (c) frequency dependences of tan δ for linear PLA precursor and LCB PLA samples at 180 °C.

LCB PLA samples showed a slower relaxation process than the linear PLA precursor. The relaxation mechanism for the LCB PLA samples changed from the simple “reptation mode” for linear entangled macromolecules to the slow “arm retraction mode” for LCB macromolecules, as a result of the topological constraints of the LCB chains. Moreover, the LCB PLA samples exhibited an additional characteristic relaxation time (τB) in addition to the linear one (τL). The long relaxation time 1154



Article

earlier the strain hardening occurred. A similar behavior was also found for LCB isotactic polypropylene (iPP) samples with a tree-like molecular structure, which were prepared by electron beam irradiation at high irradiation doses.52 Interestingly, LCBPLA1 exhibited a strain-rate dependence of the elongational flow behavior, that is, a change from a strain softening at lower strain rates (0.05 and 0.1 s−1) to strain hardening at higher strain rates (0.3 and 0.5 s−1). It was noticed that strain hardening could be observed only for polymers with sufficient LCBs. This fact is attributed to the fewer entanglements of the diluted LCB molecules. In our case, strain hardening could be observed for the LCB PLA sample prepared with only 0.2 wt % TMPTA (LCB-PLA1 at the higher strain rates). A probable reason for the low threshold of LCB content lies in the unique structure of LCB PLA samples in this study. The LCB structures were introduced by the branching reactions between the γ-irradiated macroradicals, which were randomly located on the PLA backbone chains, and the CC groups on TMPTA. The average chain lengths between branch points were well above the critical entanglement molecular mass (8000 g/mol for PLA) and were long enough to entangle with the linear PLA chains. In addition, the polydispersity or especially the higher molecular mass components can principally influence the elongational flow viscosity.53,54 In the case of LCB-PLA1, the LCB molecules had a higher molecular mass than their linear PLA precursor, which broadened the molecular mass distribution, well reflected by the elongational rheology. For quantitative evaluation of the strain-hardening behavior, the so-called strain-hardening coefficient, χE, was obtained as

Figure 5. Weighted relaxation spectra of linear PLA precursor and LCB PLA samples at 180 °C. The vertical dashed lines represent the time scale covered in the measurements. Inset: Spectra at the full scale.

is ascribed to the relaxation of the branched chains. The short relaxation time for LCB PLA samples was close to that of the linear PLA precursor, indicating that the linear structure still existed in the LCB PLA samples. The bimodal distribution of the chain architectures also was reflected in the column fraction process during the SEC-MALLS measurements. Note that the longer relaxation time of LCB-PLA4 was significantly larger than the maximum relaxation time that could be determined in the experiments (see the plot in the inset of Figure 5) and the short relaxation time of the linear chains could not be distinguished, indicating denser entanglements due to the higher content of long-chain branched structures. A similar double relaxation time distribution was also reported by WoodAdams and Costeux for long-chain branched PE.48 3.3. Uniaxial Elongational Flow Behaviors for LCB PLA Samples. An examination of the extensional flow characteristics was performed using an ARES G2 rheometer equipped with an extensional viscosity fixture (EVF) to evaluate the mechanical behaviors of the LCB PLA samples under stretching in the molten state. When the elongational viscosity is recorded as a function of specimen strain, a sudden increase in viscosity at a certain strain is called strain-hardening behavior, which has been reported for several LCB polymers.49,50 The onset strains of strain-hardening are dependent on the branching degree or the content of the high-molecular-mass fraction. Such a strain hardening is important for polymer processing where high melt strength is required, such as high-speed fiber spinning and film blowing.30,51 Uniaxial elongational flow measurements at different constant elongational rates were performed at 170 °C for the linear PLA precursor and LCB PLA samples. The maximum achievable Hencky strains for these samples were in the range from 1.4−3.9, depending on the strain rate and LCB content. Figure 6 shows the changes in elongational flow viscosity as a function of time at elongational rates ranging from 0.05 to 0.5 s−1. The linear PLA0 did not exhibit any strain hardening, and the sample broke prematurely before the maximum extension strain of 3.9 could be achieved in the measurements. In contrast, the LCB PLA samples (LCB-PLA2, LCB-PLA3, and LCB-PLA4) resisted the elongational deformation up to the maximum strain of 3.9 without failure and displayed strainhardening behavior. The higher the strain rate applied, the

χE (t ) =

ηE+(t , ε)̇ 3η0+(t )

(1)

η+0 (t)

where is the time-dependent shear viscosity in the linear range of deformation, which is determined using the discrete relaxation spectrum according to the equation N

η0+(t ) = 3 ∑ λiGi(1 − e−t / λi) i=1

(2)

where Gi is the modulus corresponding to the relaxation time λi. Note that the parameters Gi and λi were obtained by calculations from the G′ and G″ data using the standard nonlinear regression method in the Trios Software (TA Instruments, New Castle, DE). The changes in strain-hardening coefficient, χE, as a function of Hencky strain rate, ε̇, for LCB PLA samples at a Hencky strain of 2.7 are shown in Figure 7. It can be clearly seen that the level of strain hardening increased with increasing LCB content and there existed different strain rate dependences. For LCB-PLA1, the strain-hardening coefficient increased from values below 1 to values above 1 with increasing strain rate, which implies that the elongational flow behavior changed from strain softening to strain hardening. LCB-PLA2 showed a monotonic increase in strain-hardening coefficient with increasing strain rate, and a similar behavior was also found for chemically modified LCB PLA samples and low-density polyethylene (LDPE), which have a high number of long-chain branches and a tree-like molecular structure.31,55 It was interesting to find that, when more TMPTA was incorporated (for LCB-PLA3 and LCBPLA4), the strain-hardening coefficient, χE, passed through a maximum with an increase in magnitude. Similar behavior has also been reported by Krause et al. for electron-beam-irradiated 1155



Article

Figure 6. Changes in elongational viscosity as a function of time at different elongational flow rates (0.05, 0.1, 0.3, and 0.5 s−1) at 170 °C for (a) PLA0, (b) LCB-PLA1, (c) LCB-PLA2, (d) LCB-PLA3, and (e) LCB-PLA4.

poor foaming behavior at both 70 and 90 °C. The foam fabricated at 70 °C had a poor cell structure and obvious unfoamed regions. At the higher foaming temperature of 90 °C, the walls between the foaming bubbles were mostly ruptured as a result of intense shearing during the fast bubble development, and the collapse of neighboring bubbles led to an increase in the cell sizes. The melt strength of linear PLA precursor was not strong enough to withstand the internal pressure during the foaming process, resulting in significant cell collapse and wall rupture. On the contrary, LCB-PLA4 demonstrated much improved foaming performance in terms of cell morphology such as uniformity in cell sizes and closed cell walls at both foaming temperatures of 70 and 90 °C, as shown in Figure 8b,d. As a result of the high melt strength brought by the introduction of LCB structures, the quick expansion of CO2 gas

polypropylene samples at irradiation temperatures between 110 and 190 °C, which exhibited increasing numbers of branches with decreasing branch lengths for increasing irradiation temperatures.56 From this point of view, the increasing TMPTA amount incorporated in the irradiation process for PLA might have effects similar to the elevated irradiation temperature for the LCB PP system in producing more branching points and decreasing the branch lengths. 3.4. Improved Foaming Performance for LCB PLA Samples. The effects of the improved melt strength of the LCB PLA samples on the foaming behavior were investigated by batch foaming process using compressed carbon dioxide (CO2) as the foaming reagent. Panels a and c of Figure 8 show the cell morphologies of foamed PLA0 obtained at foaming temperatures of 70 and 90 °C, respectively. PLA0 exhibited 1156



Article

Figure 7. Changes in strain-hardening coefficient, χE, as a function of Hencky strain rate, ε̇, for LCB PLA samples at a Hencky strain of 2.7.

Figure 9. SEM micrographs of cryo-fractured (a) PLA0 and (b) LCBPLA4 foamed at 70 °C with higher magnification and (c) photographs of LCB-PLA4 samples before foaming (clear sample in the left image) and after foaming (white sample in the right image).

in the irradiation process upon the addition of TMPTA, and the LCB content and degree of branching in LCB PLA samples increased with increasing amount of TMPTA. The linear rheological properties of the LCB PLA samples were found to be much different from those of the linear PLA precursor, showing a higher complex viscosity, |η*(ω)|, and a higher storage modulus, G′, at low frequencies, upturning at high viscosities in the Cole−Cole plot, an appearance of a plateau in the tan δ−ω plot, and additional much longer relaxation time in the weighted relaxation spectra. More important, the LCB PLA samples exhibited a pronounced strain-hardening behavior with extensional flow rate dependence, which can be ascribed to the enhanced melt strength. Furthermore, the enhanced melt strength of the LCB PLA samples contributes to their improved foaming performance as compared with that of the linear PLA precursor.

Figure 8. SEM micrographs of cryo-fractured (a,c) PLA0 and (b,d) LCB-PLA4 foamed at (a,b) 70 and (c,d) 90 °C. The scale bars in the micrographs represent 200 μm.

■

in LCB-PLA4 could not break the walls of neighboring bubbles that help generate foams with closed cell morphologies and thicker cell walls as compared with that of the foamed PLA0 (Figure 9a,b). It can be concluded that the foaming properties of the LCB-PLAs are much better than that of linear PLA precursor, and such improved foaming properties can be ascribed to the LCB structures and the improved melt strength. Moreover, Figure 9c shows photographs of LCB-PLA4 before foaming (a clear transparent sample) and after foaming (a white sample with an expansion ratio of 2−3) at 70 °C. The solidstate foaming strategy for LCB-PLA materials using compressed carbon dioxide (CO2) as the foaming reagent can be used to produce foams with closed cell structures and smooth surface, which can surely expand the application fields of PLA materials for thermal insulators and disposable food packages.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 0551-63607703. Fax: +86 0551-63607703. E-mail: [email protected]. *Tel.: +86 0551-63607703. Fax: +86 0551-63607703. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Z.W. acknowledges financial support from the National Science Foundation of China (Grant 21174139) and National Basic Research Program of China (Grant 2012CB025901). Dr. Wentao Zhai at Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, is acknowledged for providing the use of the foaming facility, and Dr. ChenYang Liu at Institute of Chemistry, Chinese Academy of Sciences, is acknowledged for providing access to the extensional rheological facility.

4. CONCLUSIONS Long-chain branched polylactides (LCB PLA samples) with high melt strength were prepared by applying γ-radiation with addition of the trifunctional monomer TMPTA. The degradation of the PLA chains occurred during irradiation by γ-rays without addition of TMPTA, as verified by the viscosity curves and van Gurp−Palmen plots. The SEC-MALLS measurements revealed that the LCB structures were formed

■

REFERENCES

(1) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Polylactic acid technology. Adv. Mater. 2000, 12, 1841−1846.

1157



Article

(25) Gottschalk, C.; Frey, H. Hyperbranched polylactide copolymers. Macromolecules 2006, 39, 1719−1723. (26) Jikei, M.; Suzuki, M.; Itoh, K.; Matsumoto, K.; Saito, Y.; Kawaguchi, S. Synthesis of hyperbranched poly(L-lactide)s by selfpolycondensation of AB2 macromonomers and their structural characterization by light scattering measurements. Macromolecules 2012, 45, 8237−8244. (27) Carlson, D.; Dubois, P.; Nie, L.; Narayan, R. Free radical branching of polylactide by reactive extrusion. Polym. Eng. Sci. 1998, 38, 311−321. (28) Dean, K. M.; Petinakis, E.; Meure, S.; Yu, L.; Chryss, A. Melt strength and rheological properties of biodegradable poly(lactic aacid) modified via alkyl radical-based reactive extrusion processes. J. Polym. Environ. 2012, 20, 741−747. (29) Corre, Y. M.; Duchet, J.; Reignier, J.; Maazouz, A. Melt strengthening of poly(lactic acid) through reactive extrusion with epoxy-functionalized chains. Rheol. Acta 2011, 50, 613−629. (30) Mihai, M.; Huneault, M. A.; Favis, B. D. Rheology and extrusion foaming of chain-branched poly(lactic acid). Polym. Eng. Sci. 2010, 50, 629−642. (31) Liu, J. Y.; Lou, L. J.; Yu, W.; Liao, R. G.; Li, R. M.; Zhou, C. X. Long chain branching polylactide: Structures and properties. Polymer 2010, 51, 5186−5197. (32) Wang, L.; Jing, X.; Cheng, H.; Hu, X.; Yang, L.; Huang, Y. Rheology and crystallization of long-chain branched poly(L-lactide)s with controlled branch length. Ind. Eng. Chem. Res. 2012, 51, 10731− 10741. (33) You, J.; Lou, L.; Yu, W.; Zhou, C. The preparation and crystallization of long chain branching polylactide made by melt radicals reaction. J. Appl. Polym. Sci. 2013, 129, 1959−1970. (34) Shin, B. Y.; Han, D. H.; Narayan, R. Rheological and thermal properties of the PLA modified by electron beam irradiation in the presence of functional monomer. J. Polym. Environ. 2010, 18, 558− 566. (35) Wang, Y. B.; Yang, L.; Niu, Y. H.; Wang, Z. G.; Zhang, J.; Yu, F. Y.; Zhang, H. B. Rheological and topological characterizations of electron beam irradiation prepared long-chain branched polylactic acid. J. Appl. Polym. Sci. 2011, 122, 1857−1865. (36) Auhl, D.; Stadler, F. J.; Münstedt, H. Comparison of molecular structure and rheological properties of electron-beam- and gammairradiated polypropylene. Macromolecules 2012, 45, 2057−2065. (37) Fang, H. G.; Zhang, Y. Q.; Bai, J.; Wang, Z. K.; Wang, Z. G. Bimodal architecture and rheological and foaming properties for gamma-irradiated long-chain branched polylactides. RSC Adv. 2013, 3, 8783−8795. (38) Othman, N.; Acosta-Ramirez, A.; Mehrkhodavandi, P.; Dorgan, J. R.; Hatzikiriakos, S. G. Solution and melt viscoelastic properties of controlled microstructure poly(lactide). J. Rheol. 2011, 55, 987−1005. (39) Babanalbandi, A.; Hill, D. J. T.; Odonnell, J. H.; Pomery, P. J.; Whittaker, A. An electron spin resonance study on gamma-irradiated poly(L-lactic acid) and poly(D,L-lactic acid). Polym. Degrad. Stab. 1995, 50, 297−304. (40) Babanalbandi, A.; Hill, D. J. T.; Whittaker, A. K. Volatile products and new polymer structures formed on 60Co γ-radiolysis of poly(lactic acid) and poly(glycolic acid). Polym. Degrad. Stab. 1997, 58, 203−214. (41) Nugroho, P.; Mitomo, H.; Yoshii, F.; Kume, T. Degradation of poly(L-lactic acid) by gamma-irradiation. Polym. Degrad. Stab. 2001, 72, 337−343. (42) van Gurp, M.; Palmen, J. Time−temperature superposition for polymeric blends. Rheol. Bull. 1998, 67, 5−8. (43) Trinkle, S.; Friedrich, C. Van Gurp−Palmen-plot: A way to characterize polydispersity of linear polymers. Rheol. Acta 2001, 40, 322−328. (44) Trinkle, S.; Walter, P.; Friedrich, C. Van Gurp−Palmen plot IIClassification of long chain branched polymers by their topology. Rheol. Acta 2002, 41, 103−113.

(2) Jamshidian, M.; Tehrany, E. A.; Imran, M.; Jacquot, M.; Desobry, S. Poly-lactic acid: Production, applications, nanocomposites, and release studies. Compr. Rev. Food Sci. Food Saf. 2010, 9, 552−571. (3) Williams, C. K.; Hillmyer, M. A. Polymers from renewable resources: A perspective for a special issue of polymer reviews. Polym. Rev. 2008, 48, 1−10. (4) Gross, R. A.; Kalra, B. Biodegradable polymers for the environment. Science 2002, 297, 803−807. (5) Palade, L. I.; Lehermeier, H. J.; Dorgan, J. R. Melt rheology of high L-content poly(lactic acid). Macromolecules 2001, 34, 1384−1390. (6) Dorgan, J. R.; Janzen, J.; Clayton, M. P.; Hait, S. B.; Knauss, D. M. Melt rheology of variable L-content poly(lactic acid). J. Rheol. 2005, 49, 607−619. (7) Yu, L.; Dean, K.; Li, L. Polymer blends and composites from renewable resources. Prog. Polym. Sci. 2006, 31, 576−602. (8) Lim, L. T.; Auras, R.; Rubino, M. Processing technologies for poly(lactic acid). Prog. Polym. Sci. 2008, 33, 820−852. (9) Simões, C. L.; Viana, J. C.; Cunha, A. M. Mechanical properties of poly(ε-caprolactone) and poly(lactic acid) blends. J. Appl. Polym. Sci. 2009, 112, 345−352. (10) Zhang, J.; Li, G.; Su, Y.; Qi, R.; Ye, D.; Yu, J.; Huang, S. Highviscosity polylactide prepared by in situ reaction of carboxyl-ended polyester and solid epoxy. J. Appl. Polym. Sci. 2012, 123, 2996−3006. (11) Wu, D.; Zhang, Y.; Zhang, M.; Zhou, W. Phase behavior and its viscoelastic response of polylactide/poly(ε-caprolactone) blend. Eur. Polym. J. 2008, 44, 2171−2183. (12) Lu, J.; Qiu, Z.; Yang, W. Fully biodegradable blends of poly(Llactide) and poly(ethylene succinate): Miscibility, crystallization, and mechanical properties. Polymer 2007, 48, 4196−4204. (13) Xu, Z. H.; Niu, Y. H.; Yang, L.; Xie, W. Y.; Li, H.; Gan, Z. H.; Wang, Z. G. Morphology, rheology and crystallization behavior of polylactide composites prepared through addition of five-armed star polylactide grafted multiwalled carbon nanotubes. Polymer 2010, 51, 730−737. (14) Weng, W.; Hu, W.; Dekmezian, A. H.; Ruff, C. J. Long chain branched isotactic polypropylene. Macromolecules 2002, 35, 3838− 3843. (15) Fetters, L. J.; Kiss, A. D.; Pearson, D. S.; Quack, G. F.; Vitus, F. J. Rheological behavior of star-shaped polymers. Macromolecules 1993, 26, 647−654. (16) Wood-Adams, P. M.; Dealy, J. M.; deGroot, A. W.; Redwine, O. D. Effect of molecular structure on the linear viscoelastic behavior of polyethylene. Macromolecules 2000, 33, 7489−7499. (17) Kapnistos, M.; Vlassopoulos, D.; Roovers, J.; Leal, L. G. Linear rheology of architecturally complex macromolecules: Comb polymers with linear backbones. Macromolecules 2005, 38, 7852−7862. (18) Munstedt, H. Rheological properties and molecular structure of polymer melts. Soft Matter 2011, 7, 2273−2283. (19) Tian, J.; Yu, W.; Zhou, C. Crystallization kinetics of linear and long-chain branched polypropylene. J. Macromol. Sci. B: Phys. 2006, 45, 969−985. (20) Zhao, W.; Huang, Y.; Liao, X.; Yang, Q. The molecular structure characteristics of long chain branched polypropylene and its effects on non-isothermal crystallization and mechanical properties. Polymer 2013, 54, 1455−1462. (21) Tian, J.; Yu, W.; Zhou, C. Crystallization behaviors of linear and long chain branched polypropylene. J. Appl. Polym. Sci. 2007, 104, 3592−3600. (22) Agarwal, P. K.; Somani, R. H.; Weng, W.; Mehta, A.; Yang, L.; Ran, S.; Liu, L.; Hsiao, B. S. Shear-induced crystallization in novel long chain branched polypropylenes by in situ rheo-SAXS and -WAXD. Macromolecules 2003, 36, 5226−5235. (23) Arvanitoyannis, I.; Nakayama, A.; Kawasaki, N.; Yamamoto, N. Novel star-shaped polylactide with glycerol using stannous octoate or tetraphenyl tin as catalyst: 1. Synthesis, characterization and study of their biodegradability. Polymer 1995, 36, 2947−2956. (24) Kricheldorf, H. R.; Hachmann-Thiessen, H.; Schwarz, G. Telechelic and star-shaped poly(L-lactide)s by means of bismuth(III) acetate as initiator. Biomacromolecules 2004, 5, 492−496. 1158



Article

(45) Podzimek, S. Size exclusion chromatography. In Light Scattering, Size Exclusion Chromatography and Asymmetric Flow Field Flow Fractionation; John Wiley & Sons, Inc.: New York, 2011; pp 99−206. (46) García-Franco, C. A.; Srinivas, S.; Lohse, D. J.; Brant, P. Similarities between gelation and long chain branching viscoelastic behavior. Macromolecules 2001, 34, 3115−3117. (47) Li, S.; Xiao, M.; Wei, D.; Xiao, H.; Hu, F.; Zheng, A. The melt grafting preparation and rheological characterization of long chain branching polypropylene. Polymer 2009, 50, 6121−6128. (48) Wood-Adams, P.; Costeux, S. Thermorheological behavior of polyethylene: Effects of microstructure and long chain branching. Macromolecules 2001, 34, 6281−6290. (49) Stange, J.; Uhl, C.; Münstedt, H. Rheological behavior of blends from a linear and a long-chain branched polypropylene. J. Rheol. 2005, 49, 1059−1079. (50) Gabriel, C.; Münstedt, H. Strain hardening of various polyolefins in uniaxial elongational flow. J. Rheol. 2003, 47, 619−630. (51) Han, C. D.; Lamonte, R. R. Studies on melt spinning. I. Effect of molecular structure and molecular weight distribution on elongational viscosity. J. Rheol. 1972, 16, 447−472. (52) Auhl, D.; Stange, J.; Münstedt, H.; Krause, B.; Voigt, D.; Lederer, A.; Lappan, U.; Lunkwitz, K. Long-chain branched polypropylenes by electron beam irradiation and their rheological properties. Macromolecules 2004, 37, 9465−9472. (53) Gabriel, C.; Kaschta, J.; Münstedt, H. Influence of molecular structure on rheological properties of polyethylenes. Rheol. Acta 1998, 37, 7−20. (54) Langouche, F.; Debbaut, B. Rheological characterisation of a high-density polyethylene with a multi-mode differential viscoelastic model and numerical simulation of transient elongational recovery experiments. Rheol. Acta 1999, 38, 48−64. (55) Malmberg, A.; Gabriel, C.; Steffl, T.; Münstedt, H.; Löfgren, B. Long-chain branching in metallocene-catalyzed polyethylenes investigated by low oscillatory shear and uniaxial extensional rheometry. Macromolecules 2002, 35, 1038−1048. (56) Krause, B.; Voigt, D.; Häußler, L.; Auhl, D.; Münstedt, H. Characterization of electron beam irradiated polypropylene: Influence of irradiation temperature on molecular and rheological properties. J. Appl. Polym. Sci. 2006, 100, 2770−2780.

1159


Preparation and Characterization of High-Melt-Strength Polylactide

Recommend Documents