Chapter 4 - Mechanical analysis of polymers

Understanding how molecular weight distribution (MWD) influences the mechanical and fracture behaviour of additively manufactured PA12 is essential for establishing reliable process–structure–property relationships. An approach to minimize anisotropy has been to specifically target the feedstock materials with the goal of enhancing interlayer adhesion. In this study, polyamide 12 (PA12) grades of MWD were investigated to determine the effect of MWD on the interlayer bond formation and anisotropy of the printed samples. Three different grades of PA12 were printed into tensile coupons and double cantilever beam (DCB) specimens and the mechanical properties were assessed to study interlayer bond strength using standard fracture toughness technique. A significant improvement in mechanical properties and fracture toughness were observed with higher print temperatures attributed to strong interlayer adhesion owing to longer interlayer bonding times (verified through the Lumped Capacity model and thermocouple data). Fracture toughness assessment also demonstrated that amongst samples printed at the same temperature, PA12 with a lower average MWD provided the highest fracture toughness (G 1C ) values, attributed to the faster diffusion of short polymer chains across the interlayer interface. Tailoring the MWD of feedstock polymers utilized in 3D printing as a strategy to optimize performance and lower anisotropy was demonstrated to be a feasible pathway. These findings were utilized by developing a PA12 blend of broad MWD which provided the highest overall mechanical properties and G 1c , demonstrating the usefulness of incorporating a high proportion of fast diffusing short polymer chains in the development of polymeric feedstock materials. Additionally, for large scale 3D printing applications, where maintaining high chamber temperatures is impractical, these results suggest that optimizing MWD can be an alternative strategy to improve interlayer adhesion and mechanical properties without relying solely on elevated print temperatures.

The applicability of tailoring polymer structure to meet power cable application requirements is demonstrated. Minimization of the high and low molecular weight tails of the molecular weight distribution (MWD) and uniform distribution of comonomer in polyolefins is highly desirable for achieving optimum properties for power cable applications. Polymer properties affected by molecular weight (MW), MWD, and composition distribution (CD) are examined. The polymer structure properties are then related to power cable application performance criteria. Particular emphasis is placed on the properties of narrow MWD narrow CD resins and the combination of these narrowly distributed resins. These polymer properties are then related to performance in wire and cable coating applications, i.e. crosslinkability, filler acceptance, tree retardancy, etc. The electrical and mechanical properties with regard to wire and cable applications are also discussed. >

Modelling the fluid phase behaviour of polydisperse polyethylene blends in hydrocarbons using the modified Sanchez–Lacombe equation of state

The structural changes in the hydrolyzed polyacrylamide (PAM) under different saline conditions show significant effects on the rheological properties. At the higher shear rates, these conformational changes cannot be discerned in the shear rheological field but the extensional rheology can probe the polymer's microstructure can provide a better description. In this paper, two types of polymers: co‐hydrolyzed PAM and post‐hydrolyzed PAM were tested under different saline conditions. The polymer conformation results were interpreted to ascertain the underlying mechanisms of polymer and cation interactions and their effect on the extensional properties. The results showed that the interactions of polymer chains with divalent cations lead to the formation of the cross‐linked chelating structures corresponded to the narrow molecular weight distribution (MWD) with a lower radius of Gyration (Rg). However, the uniform carboxylate ion density across the co‐hydrolyzed PAM restricted shrinkage of the polymer chains after interactions with cations showed the high strain hardening due to polymer chain entanglement at high Hencky strain. Whereas, monovalent cation interactions with the carboxylate groups resulted in the monodentate structures that sustained the broader MWD that explains the observed higher strain hardening and extensional relaxation time.

Polymeric materials are used in several parts because their flexibility, low cost and excellent in processing. It is important to understand the some properties of polymeric materials for optimized design. The properties of polymeric materials are strongly influenced by meso-scale (10-9 10-3 m) structure such as entanglement, molecular weight distribution, and orientation, etc. But it is difficult to investigate the relationship by experimental methods because the structures are micro scale. For this reason, the detail relationship between the properties and meso-scale structure is not clear. Recently, computing power increases and increases, therefore it is possible to calculate behaviors of molecular chains. In this study, it is aimed to clear this relationship by the simulation. Mechanical properties are one of the important properties of polymeric materials. Therefore, the relationship between the mechanical properties and meso-scale properties has been investigated. But optical property is also important as well as mechanical property. In this study, we propose simulation method to investigate the relationship between the optical property and meso-scale structure. Polymeric materials are modeled by networks of molecular chain considered the meso-scale structure. In addition, polymeric materials are birefringence material. When the polymeric materials are elongated, refraction indexes of several directions become different values. A modeling of this change of refraction index is proposed. From these modeling, the light intensity distribution in a cross-section of square optical-waveguide is obtained. Then, the relationship between the change of structure of molecular chain scale and light intensity is investigated.

A multi-scale method for modeling degradation of bioresorbable polyesters

The manipulation of a polymer’s properties without altering its chemical composition is a major challenge in polymer chemistry, materials science and engineering. Although variables such as chemical structure, branching, molecular weight and dispersity are routinely used to control the architecture and physical properties of polymers, little attention is given to the often profound effect of the breadth and shape of the molecular-weight distribution (MWD) on the properties of polymers. Synthetic strategies now make it possible to explore the importance of parameters such as skew and the higher moments of the MWD function beyond the average and standard deviation. In this Review, we describe early accounts of the effect of MWD shape on polymer properties; discuss synthetic strategies for controlling MWD shape; describe current endeavours to understand the influence of MWD shape on rheological and mechanical properties and phase behaviour; and provide insight into the future of using MWDs in the design of polymeric materials. Controlling polymerization is required to fully understand and tune the function of a polymer. Alongside variables such as dispersity, molecular weight and chemical composition, the shape of the molecular-weight distribution has substantial influence on polymer properties. In this Review, we discuss methods to systematically control the shape of a polymer’s molecular-weight distribution, as well as explore the profound effects of shape on polymer properties.

The MWD is probably the single most fundamental property of a polymer (see Molecular Weight Distribution and Mechanical Properties). Depending on the application, MW of polymer materials can encompass an extremely large range extending from several hundreds (paints, coatings, functionalised polymers) to a few million daltons (UHMWPE, high performance fibres). Unlike in the case of small molecules, the assignment of an unequivocal MW to a macromolecular material is not straightforward. Due to the kinetics of polymerization, no synthetic polymer has a unique MW.KeywordsMolecular Weight DistributionUniversal CalibrationSteric ExclusionEnthalpic InteractionHigh Performance FibreThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Mechanical properties of recycled PP/rubber/filler systems were studied and mechanisms were discussed on the basis of molar mass and molar mass distribution, morphologies, and thermal behavior. Characteristics of recycled PP/rubber/filler depend on degradation of PP and additional components. Recycling processes decreased impact strength, but it seemed not to affect tensile properties significantly. Aging with UV and moisture affected mechanical properties by some degree of PP chain scissions and ethylene chain crosslinking in EP rubber. Inorganic filler acted as a crack center and accelerated aging degradation. Paint decreased mechanical properties significantly by introducing stress concentrations. Recycle processing decreased the molar mass and increased the molar mass distribution of PP. Aging with UV decreased crystallization temperature and increased crystallization half‐time by decreasing nucleation. © 1995 John Wiley & Sons, Inc.

The potential effects of microplastic particle exposure on environmental organisms has sparked intense research activities. Various studies have been conducted, however on a limited set of mostly pristine polymer materials. In parallel to the ongoing research activities, it is discussed to include non-natural polymers into the registration process under REACH. Currently, non-natural polymers are exempted from registration, based on the general assumption of being non-hazardous due to their high molecular weight. In addition, the extensive number of polymers exceeding the registration capacities was mentioned as a reason for exemption. Hence, relevant polymers requiring a registration shall be selected according to specific criteria that help to identify those with a concern for hazardous effects. In the line of these developments we here present the results of a systematic ecotoxicity testing of 16 microplastic particles of different polymer composition, part of which are micronized polymer powders used in 3D printing (intentionally produced primary microplastic, losing particle shape by the 3D printing). All polymer materials were systematically varied and extensively characterised with regard to their properties (e.g. particle size, cross-linking, molar mass distribution, end groups, reactivity). Most of the polymers exerted toxicity in 48 h acute Daphnia magna immobilisation assay and 72 h chronic Raphidocelis subcapitata growth inhibition assay, except one PA and one HDPE material showing toxicity in D. magna, and one HDPE material showing toxicity in green algae. From these results we conclude that none of the microplastic particles studied here, independent of their polymer properties, give raise to concern for hazardous effects.

The fact that a polymer consists of a number of chains of different lengths, each in turn consisting of a series of monomer units, means that the motion of one part of the polymer chain will profoundly affect the motion of other parts. Hence, for a given polymer, a description of microscopic processes occurring under a given flow field depends on hypotheses regarding the molecular structure and mechanisms of flow in the polymer. Today, it is well-known, gained from practical experience, that the molecular weight, the molecular weight distribution, and the degree of long-chain branching influence the rheological properties of polymeric liquids. Therefore, a better understanding of the relationship between molecular parameters and rheological properties is very important from the standpoints of both polymer synthesis and polymer processing. However, the theoretical development of this aspect of the problem is far from complete, although some important progress has been made. In the preceding chapter, we discussed the viscoelastic behavior of polymeric liquids from the phenomenological point of view, without associating the significance of theoretical predictions to molecular origin(s). Specifically, we have seen that the rheological equations of state contain parameters that vary from polymer to polymer. Since it has amply been demonstrated by experiment that the extent of a particular viscoelastic behavior is greatly influenced by the molecular parameters, such as molecular weight, molecular weight distribution, and the degree of long-chain branching, predictions of any viscoelastic behavior of polymers on the basis of phenomenological theory is of very limited use to either control the quality of polymers produced or improve the performance of polymers, unless the parameters appearing in various continuum constitutive equations are related to molecular parameters.

The molecular weight distribution (MWD) of crosslinked polymer molecules formed during polymeric network formation is the sum of the fractional MWDs containing 0, 1, 2, 3, … crosslinkages. The MWD for polymer molecules containing 𝓀 crosslinkages is investigated for the random crosslinking of polymer chains whose initial MWD is given by the Schulz‐Zimm distribution. For a very narrow initial MWD, each fractional MWD with 𝓀 = 0, 1, 2, … is independent and a multimodal distribution is obtained for the whole distribution. When the initial MWD is uniform, the average crosslinking density within the polymer fraction whose degree of polymerization is r, ρr is simply given by ρr = ρgel,c – 2/r irrespective of the extent of crosslinking reaction where ρgel,c is the crosslinking density within gel fraction at the gel point. On the other hand, the MWDs with 𝓀 crosslinkages overlap each other with different 𝓀 values significantly for the broader initial distributions, and ρr increases with the progress of crosslinking reactions. The value of ρr increases with increasing r but levels off asymptotically at large r. The average crosslinking density of polymer molecules containing 𝓀 crosslinkages ρ𝓀 is an increasing function of k but soon reaches a plateau; sooner for the broader initial MWDs. For 𝓀 ≥ 1, ρ𝓀 is always larger than the average crosslinking density of the whole reaction system ρ in the pregelation period, i.e., in terms of the crosslinking density, the difference between polymer molecules with and without crosslinkage is most significant. In general, the average crosslinking density ρ, which is convenient to use in describing the nature of the whole reaction system, cannot be considered as a characteristic degree of crosslinking for polymer molecules containing at least one crosslinkage. Consideration of the bivariate distribution of r and k reveals important aspects of the polymeric network formation that have been obscured in the conventional theories in which the averages including linear polymers are solely considered. © 1995 John Wiley & Sons, Inc.

In this work, we report the synthesis and characterization of non‐isocyanate polyhydroxyurethanes (PHUs) derived from resorcinol bis(cyclocarbonate) (RCC). The PHUs were obtained through polyaddition between RCC and two aliphatic diamines, namely isophoronediamine and Priamine 1075 (a commercial vegetal based diamine), selected due their eco‐friendliness, under mild conditions, without the use of toxic isocyanates and catalysts. The resulting materials were characterized by FTIR, NMR spectroscopy, GPC, DSC, and TGA to evaluate their chemical structure, molecular weight distribution, and thermal properties. Then, the processability of the synthesized PHUs was assessed and further functional characterizations, such as mechanical and viscoelastic properties, swelling and water contact angle tests, were performed. Finally, the RCC based PHUs were tested as potential adhesives for aluminium, polymer composite, and wood substrates. All the results highlight that RCC based PHUs are promising sustainable alternatives to fossil‐based polyurethanes.

Polystyrene and butadiene—styrene copolymers (SBR) were parepared in emulsion systems with a homologous series of commercial mercaptan modifiers. The molecular weight distribution (MWD) of the sets of polymers changed in a consistant manner when the regulating index of the mercaptan was relatively low. However the shape of the MWD curves appeared distorted in comparison to theoretical curves when the modifier depleted rapidly and when divinyl-benzene was present in the system. The divergence from the theoretical curve is attributed to a higher degree of branching in the high molecular weight fractions. Differences in MWD of SBR made with n- and tert-dodecyl mercaptans was marked. Notable differences were also found for SBR 1500 samples from the industry at random, but only slight differences were seen in a set of SBR 1503 samples. This study shows how the MWD of polymers prepared in emulsions can be varied simply by use of modifiers with different regulating indexes.

Hyphenating high temperature high performance liquid chromatography (HT-HPLC) with high temperature size exclusion chromatography (HT-SEC) (high temperature two dimensional liquid chromatography (HTHPLC x HT-SEC or HT 2D-LC)) leads to an isocratic elution in the second dimension, which in turn enables to use IR detector (quantitative detection) for monitoring the eluting polymers. Experimental data obtained from HT 2D-LC with IR detector are usually presented as contour plots, which can be mathematically described in matrices. Quantitative data about chemical composition, molar mass and concentration of all the segments, which are present in a polymer, can be obtained, after calibrating the HPLC separation (HPLC elution volume vs chemical composition), SEC separation (SEC separation vs molar mass) and response of the IR detector (IR response vs mass of the polymer). A new procedure based on subtraction and addition of matrices is described, which enables quantitative comparison of different polymer materials. This procedure enables to determine, which components are present in both materials (i.e., identical components or segments) and which are present only in one from both the materials (i.e., unique segments). Moreover, molar mass distribution, as well as chemical composition distribution of both identical and unique segments is evaluated from experimental data. The procedure was applied on two different ethylene-propylene copolymer samples. Polyolefins J (2016) 3: 119-133