Effect of molecular weight distribution on fatigue crack propagation in poly(methyl methacrylate)

It is well established that both molecular weight (M) and its distribution (MD) affect many polymer properties such as mechanical behavior. Thus studies have shown that fatigue life is enhanced by increases in M. Research here has shown that with notched specimens fatigue crack propagation (FCP) rates are dramatically decreased by increasing M, even when the M is high enough that the static fraeture energy has essentially reached its asymptotic limit. In this study, specimens of poly(methyl methacrylate) containing either high- or low-M tails were prepared and characterized. The earlier finding that FCP rates are inversely related to average M was confirmed, but specific effects of M distribution were observed. At constant Mⁿ, a low-M tail had little effect on FCP resistance, while a high-M tail improved FCP resistance of polymers whose average M was too low for effective entanglements. Thus with high-M tails, it was possible to test specimens whose average M's were too low to permit machining. It is proposed that the effects noted are due to relative stabilization or destabilization of crazes ahead of the crack.     

Craze development in poly(methyl methacrylate) during stable fatigue crack propagation

In order to obtain a more complete understanding of failure mechanisms in glassy polymers subjected to fatigue loading conditions, craze zone dimensions (i.e., length and thickness at the crack tip) were measured simultaneously with fatigue crack propagation data in poly(methyl methacrylate) (PMMA) by optical interferometry. Since the craze shape was observed to assume a wedge-shaped configuration similar to the one described by the Dugdale plastic strip model, crazing stresses were inferred on the basis of this model. When varying the stress ratio (R = minimum load/maximum load) of the applied cyclic load in the range from 0.1 to 0.7, it was found that both craze length and craze thickness are essentially independent of the R-ratio and can be correlated in terms of the maximum stress intensity factor only. On the other hand, significant variations in craze dimensions with test frequency occurred over the range from 0.1 to 250 Hz. The results are discussed in terms of the viscoelastic nature of the material and a competition between the effects of strain rate and hysteretic heating.     

Swelling of poly(methyl methacrylate) thin films in low molecular weight alcohols

The effects of solvent size, temperature, and polymer molecular weight on the swelling of poly(methyl methacrylate) (PMMA) thin films in low molecular weight alcohols were investigated using an in situ ellipsometer. Apparent activation energies were indicative of non-Fickian diffusion, although optical data showed substantial Fickian character for swelling in methanol and moderate Fickian character in ethanol. Penetration rates were strongly dependent on the solvent molar volume for methanol, ethanol, and isopropanol, but 1-butanol and 2-pentanol had rates similar to isopropanol. The effective cross sections of these longer molecules are similar to isopropanol, and this apparently explains the similar penetration rates. The effect of polymer molecular weight (MW) on methanol penetration rates (21–27°C) was investigated with monodisperse PMMA (M_n = 6.4–40.0 × 10⁴ g/mol). A minimum at intermediate MW was observed. Isopropanol swelling rates (45–52°C) were insensitive to MW. The swelling data were also used to determine parameters for transport models that describe the swelling of thin polymer films.     

Water sorption of poly(methyl methacrylate): 1. Effects of molecular weight

A sample of poly(methyl methacrylate), PMMA, of low molecular weight (10⁴ D) took up less water (1.2%) than samples of high molecular weight (10⁶ D: 2.0%). In contrast, the uptake of water was only slightly dependent on molecular weight for samples made by radiolysis in the glassy state. It is concluded that uptake of water depends on the closer molecular packing possible in polymers of lower molecular weight. However during radiolysis in the glassy state, this potential is not fully realized because of limited mobility. In more detail, the small changes in the diffusion coefficient and uptake of water in irradiated samples were consistent with closer packing in samples with M_n ⩽ 10 000. It is concluded that molecular packing proceeds more readily below the critical molecular weight for formation of an entangled network.     

Influence of molecular weight on the fracture of poly(methyl methacrylate) (PMMA)

A model is proposed to explain the dependence of fracture parameters on the molecular weight of glassy polymers. The model assumes that the fracture event occurs in two stages; the first involves the orientation of polymer chain segments between entanglement points and the second, the fracture itself. A value has been calculated, (～0.6J m^?2), for the fracture surface energy corresponding to the lower critical molecular weight between entanglements, M=M_e. Allowing for the simplifying assumptions made in its derivation, this value is in good agreement with that found experimentally. It is proposed that, after the chain segments between entanglements crossing a plane have been fully extended, two possible mechanisms are involved; chain ‘pull-out’ up to a maximum governed by the time scale of the local fracture event, or chain scission. Using the concept of a reptating chain it is proposed that above M ～2 M_e there is a relationship between the fracture energy (γ) and the molecular weight of the form γ∝ ∝M² up to a critical value of M, above which γ is constant. It has been shown that there is some agreement with experimental relationships determined independently.     

悬浮聚合法制取不同分子量级别的聚甲基丙烯酸甲酯

采用粉状MgCO3 作为分散剂 ,悬浮聚合制取了分子量从 2 4× 10 4 ～ 2 5 4× 10 4 的聚甲基丙烯酸甲酯。考察了温度、引发剂种类和浓度、分子量调节剂、转化率对聚合物分子量的影响规律 ,用粘度法测量了聚合物聚甲基丙烯酸甲酯 (PMMA)的分子量。结果表明 :温度的升高、引发剂浓度的增大、分子量调节剂的加入都会导致分子量的减小 ,随着转化率的提高 ,聚合物的分子量增大。在同等条件下 ,引发剂过氧化苯甲酰 (BPO)聚合所得的分子量较偶氮二异丁腈 (AIBN)高。通过实验 ,得到了满足作者需求的分子量 (96× 10 4 ～ 10 0× 10 4 )的聚合物的聚合条件为 :分散剂MgCO3 用量 1% ,单体∶水相 =1∶2 5 (质量比 ) ,引发剂BPO浓度 0 5 % ,反应温度 70℃ ,反应时间 3h。     

Effect of molecular weight on rate of enthalpy relaxation of poly (methyl methacrylate) and its oligomers

The enthalpy relaxation process of methyl methacrylate oligomers and poly (methyl methacrylate) was analyzed by use of the excess enthalpy (ΔH_t), which was obtained from a differential scanning calorimetric measurement. The apparent relaxation time (τ_½) was estimated by the assumption of the single relaxation time response. The T_g of oligomers increased with an increase of molecular weight, and approached a constant value above 1 × 10⁴. The apparent relaxation time showed a similar molecular-weight dependency as T_g, and approached a constant value above 1 × 10⁴. It was expected that the molecular motion which caused the enthalpy relaxation was based on the segmental motion of main chains.     

Residual stress craze and crack formation in poly(methyl methacrylate)

Samples of poly(methyl methacrylate) with a central circular hole are compressed, and crazes form on or after unloading, provided that the strain attains or exceeds a threshold value ?_t. Crazes induced in air are transformed rapidly to cracks, but environmental crazes are more stable. These residual stress crazes form at the diameter of the hole on a plane perpendicular to the applied stress direction. In contrast, during loading, crazes form on the vertical plane containing the hole axis. Unloading crazes are relatively insensitive to changes in strain rate, whereas loading erazes have a pronounced rate dependence. Environmental residual stress crazing exhibits an apparent rate sensitivity at constant time, but the critical applied strain ?_t is essentially constant, irrespective of rate, if the sample is in contact with the environment for a sufficiently long time to ensure that the minimum ?_t is obtained. Residual stress crazes appear to initiate at the equator of the hole, and the maximum tensile residual strain, indicated by a strain gauge, occurs in this position.     

Effect of the molecular weight on the plasticization properties of poly(hexane succinate) in poly(vinyl chloride) blends

Blends of poly(vinyl chloride) (PVC) and poly(hexane succinate) (PHS) with various molecular weights were analyzed with respect to their mechanical properties, durability, and thermal stability. We found that the molecular weight of PHS played an important role in the plasticizing process, and the single glass-transition temperature (T _g) of the PVC blends measured by dynamic mechanical analysis supported the complete miscibility between PHS and PVC. The plasticizing efficiency of PHS increased as the molecular weight increased; this reflected the gradually increasing elongation at break and the decreased T _g of the PVC blend. Meanwhile, the higher molecular weight of PHS also improved the resistance of migration and thermal stability but decreased the biodegradability of the PVC blends; this was due to the strong intermolecular interactions between PHS and PVC. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47081.     

Miscibility of poly(methoxymethyl methacrylate) and poly(methylthiomethyl methacrylate) with poly(methyl methacrylate)

Summary Poly(n-propyl methacrylate) is known to be immiscible with poly(methyl methacrylate) (PMMA). However, we have found that poly(methoxymethyl methacrylate) is miscible with PMMA, indicating the importance of ether oxygen atoms in achieving miscibility. On the other hand, poly(methylthiomethyl methacrylate) is immiscible with PMMA.     

Influence of molecular weight of poly (methyl methacrylate) on its miscibility with poly (vinyl chloride) in the solution

In this work, the molecular weight effect on miscibility between poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) in cyclohexanone(CH) solutions at 30 °C was examined by the viscometric method. Three samples of PMMA were prepared by emulsion polymerization, which molecular weights were changed by tert-dodecyl-mercaptan (TDDM) content. The parameter Δb is used to predict polymer-polymer miscibility of PVC/PMMA/cyclohexanone blend. Δb values indicated that the highest molecular weight of PMMA is immiscible with PVC resin. The molecular weight of PMMA decrease with the increase of the contention of TDDM, and the contribution of miscibility PVC/PMMA blend in CH is better.     

Influence of the molecular weight of poly(methyl methacrylate) on fracture morphology in notched tension

Specimens of poly(methyl methacrylate) (PMMA) were prepared by radiolysis of a polymer from an initial viscosity-average molecular weight ($\text{M}\text{?}{}_{\mathrm{v}}$) of 1.2 × 10⁶ down to 2.6 × 10³. At a molecular weight of 1 × 10⁵, abrupt changes in fracture morphology were observed correlating with a similarly abrupt decrease in fracture surface energy (γ). As the molecular weight was decreased further, the fracture morphology resembled more that of very brittle materials such as silicate glasses. Evidence was obtained that Wallner lines can influence the disposition of ribs but not their spacing. An empirical relationship was established between functions of rib spacing (r) and fracture surface energy.     

Acoustic relaxation and infrared spectroscopic measurements of the plasticization of poly(methyl methacrylate) by water

Measurements are reported on the acoustic attenuation and velocity of dry and wet samples of poly(methyl methacrylate) over a temperature range of 5° to 70°C and over a frequency range of 5 to 35 MHz. Lowering of the glass transition temperature with increase in water content was reflected in an increase in the acoustic attenuation and a lowering of the velocity at high temperature. Comparison of the infrared spectra of wet and dry thin films indicates that water exhibits spectroscopic characteristics of isolated rather than highly clustered molecules. A study of the temperature dependence of the diffusion coefficient of water into the polymer matrix provided an activation energy for the migration process. The data suggest that water plasticizes poly(methyl methacrylate) via specific local interactions with the backbone.     

Synthesis of polymer gel electrolyte with high molecular weight poly(methyl methacrylate)-clay nanocomposite

Polymer nanocomposite gel electrolytes consisting of high molecular weight poly(methyl methacrylate) PMMA-clay nanocomposite, ethylene carbonate (EC)/propylene carbonate (PC) as plasticizer, and LiClO₄ electrolyte are reported. Montmorillonite clay was ion exchanged with a zwitterionic surfactant (octadecyl dimethyl betaine) and dispersed in methyl methacrylate, which was then polymerized to synthesize PMMA-clay nanocomposites. The nanocomposite was dissolved in a mixture of EC/PC with LiClO₄, heated and pressed to obtain polymer gel electrolyte. X-ray diffraction (XRD) of the gels indicated intercalated clay structure with d-spacings of 2.85 and 1.40 nm. In the gel containing plasticizer, the clay galleries shrink suggesting intercalation rather than partial exfoliation observed in the PMMA-clay nanocomposite. Ionic conductivity varied slightly and exhibited a maximum value of 8 × 10⁻⁴ S/cm at clay content of 1.5 wt.%. The activation energy was determined by modeling the conductivity with a Vogel-Tamman-Fulcher expression. The clay layers are primarily trapped inside the polymer matrix. Consequently, the polymer does not interact significantly with LiClO₄ electrolyte as shown by FTIR. The presence of the clay increased the glass transition temperature (Tg) of the gel as determined by differential scanning calorimetry. The PMMA nanocomposite gel electrolyte shows a stable lithium interfacial resistance over time, which is a key factor for use in electrochemical applications.     

Highly syndiotactic poly(methyl methacrylate) with narrow molecular weight distribution formed by tert-butyllithium-trialkylaluminium in toluene

Summary Polymerization of MMA with t-C₄H₉Li in toluene in the presence of trialkylaluminum(alkyl=methyl, ethyl, butyl, isobutyl, and octyl) was examined at various Al/Li ratios. Triethyl-, tributyl and trioctylaluminums worked quite effectively in the polymerization to give highly syndiotactic PMMA with narrow molecular weight distribution (MWD) at the Al/Li ratio of 3 and higher than 3. The highest syndiotacticity of 96% was attained with t-C₄H₉Li/(n-C₈H₁₇)₃Al (1/3) at –93°C. The polymerization was initiated by t-C₄H₉ anion and proceeded in a living manner. The structure of the syndiotactic PMMA is chemically identical to that of isotactic PMMA prepared with t-C₄H₉MgBr. Consequently, highly isotactic and syndiotactic PMMAs with narrow MWD and with the same chemical structure from -end to -end were obtained.     

The transport of methyl methacrylate monomer in poly(methyl methacrylate)

The absorption kinetics and equlibria of methyl methacrylate monomer into poly(methyl methacrylate) were studied over a range of penetrant activities. The interval sorption kinetics at elevated activities were determined, compared, and contrasted with the integral sorption experiments in previously unpenetrated film samples. The sorption kinetics in previously unpenetrated films were predominantly case II or relaxation controlled at high activities. A Fickian contribution to the overall kinetics was apparent at lower activities. In contrast, interval sorption, at elevated activities in previously equilibrated and plasticized samples, followed Fickian kinetics rather closely, whereas resorption, over an activity range which involved a traversal of the effective T_g, was characterized by more complicated kinetics involving a super case II mechanism at long times. These composite results reinforce the notion that the kinetics describing penetration of a single penetrant into a single polymer are extremely sensitive to the boundary condition imposed upon the polymeric sorbent.     

Influence of the molecular weight of poly(methyl methacrylate) on fracture surface energy in notched tension

Specimens of poly(methyl methacrylate) (PMMA) were prepared by the radiolysis of a polymer from an initial viscosity average molecular weight of $\text{M}\text{?}{}_{\mathrm{v}}\text{= 1.1 × 10}{}^6$ down to 1.5 × 10⁴. Corresponding values of fracture surface energy, ranging from 3.5 × 10⁵ erg/cm² to 4.5 × 10² erg/cm², were calculated from tensile data using Griffith's equation. A theoretical dependence of fracture energy on molecular weight was derived on the assumption that only molecules exceeding a critical molecular weight can contribute to the work of plastic deformation. Comparison with experimental data indicates this molecular weight to be about 1 × 10⁵. Limitations of the theoretical treatment are discussed.     

Water structure in poly(2-hydroxyethyl methacrylate): Effect of molecular weight of poly(2-hydroxyethyl methacrylate) on its property related to water

Low molecular weight poly(2-hydroxyethyl methacrylate) (polyHEMA) with a number average molecular weight (Mn) <22,600, were prepared by atom transfer radical polymerization. The molecular weight and end groups of the polyHEMA were varied, and the water content equilibrium moisture sorption and water structure were analyzed using differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). Higher water content was observed for polyHEMA with Mn < 10,000. DSC revealed that the amounts of nonfreezing water are affected neither by the molecular weight nor by the end groups of the polyHEMA. On the other hand, the amount of freezing water was affected by both the molecular weight and end groups of polyHEMA, especially for polyHEMA with Mn < 20,000. The XRD-DSC measurements showed that water in polyHEMA form hexagonal ice and that the direction of crystal growth is dependent on the molecular weight. These findings indicate that the molecular weight of polyHEMA plays a significant role in the water structure in polyHEMA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012