No / The commercial success of hydroxyapatite (HA) filled polyethylene composite has generated growing interest in improving the processability of the composite. A number of synthetic procedures and post synthesis heat treatment of HA has lead to the availability of powders with widely varying morphological features. This paper addresses the effect of morphological features of HA on the rheology and processability of an injection-moulding grade HA-HDPE composite. The results showed that low surface area HA filled composite exhibited better injection processing characteristics through improved rheological responses. The effect of reducing the surface area of the filler is to require less polyethylene to wet the filler and allows more polyethylene to be involved in the flow processes. These changes reduced the temperatures and pressures required for successful processing.

no / The shear and extensional properties of injection moulding grade hydroxyapatite¿polyethylene composites developed for orthopaedic applications have been studied. The composite was prepared without processing aids owing to concerns over the potential biological responses to such additives. The composite containing 20 vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron¿Pierce equation was found to be useful in predicting the viscosities of the composite systems. The activation energy of the composite and the unfilled polymer were equal, indicating that the 20 vol.-% system exhibits the same flow mechanism as the unfilled polymer. A qualitative assessment of extensional properties was made following Cogswell's method. The extensional stress of the unfilled polymer decreases with increasing temperature whereas the composites behave in a complex manner. For all the systems the Trouton ratios tend to increase with apparent shear rates. The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.

Yes / The shrinkage force exerted by restrained shape memory polymers can potentially be used to close cracks in structural concrete. This paper describes the physical processing and experimental work undertaken to develop high shrinkage die-drawn Polyethylene Terephthalate (PET) shape memory polymer tendons for use within a crack closure system. The extrusion and die-drawing procedure used to manufacture a series of PET tendon samples is described. The results from a set of restrained shrinkage tests, undertaken at differing activation temperatures, are also presented along with the mechanical properties of the most promising samples. The stress developed within the tendons is found to be related to the activation temperature, the cross-sectional area and to the draw rate used during manufacture. Comparisons with commercially-available PET strip samples used in previous research are made, demonstrating an increase in restrained shrinkage stress by a factor of two for manufactured PET filament samples. / Thanks must go to the EPSRC for their funding of the Materials for Life (M4L) project (EP/K026631/1) and to Costain Group PLC. for their industrial sponsorship of the project and author.

Yes / The microstructure and molecular orientation distribution over both the length and the thickness of microinjection‐molded linear low‐density polyethylene with a weld line were characterized as a function of processing parameters using small‐angle X‐ray scattering and wide‐angle X‐ray diffraction techniques. The weld line was introduced via recombination of two separated melt streams with an angle of 180° to each other in injection molding. The lamellar structure was found to be related to the mold temperature strongly but the injection velocity and the melt temperature slightly. Furthermore, the distributions of molecular orientation at different molding conditions and different positions in the cross section of molded samples were derived from Hermans equation. The degree of orientation of polymeric chains and the thickness of oriented layers decrease considerably with an increase of both mold temperature and melt temperature, which could be explained by the stress relaxation of sheared chains and the reduced melt viscosity, respectively. The level of molecular orientation was found to be lowest in the weld line when varying injection velocity, mold temperature, and melt temperature, thus providing an effective means to identify the position of weld line induced by flow obstacles during injection‐molding process. / Jilin Scientific and Technological Development Program. Grant Number: 20180519001JH; National Key R&D Program of China. Grant Number: 2018YFB0704200; National Natural Science Foundation of China. Grant Numbers: 21674119, 21790342; Newton Advanced Fellowship of Royal Society. Grant Number: NA150222

A small scale, highly accurate elongational viscometer was developed explicitly for the rheological investigation of well characterized polyethylene samples. Elongational stress growth measurements as well as dynamic shear experiments demonstrated that the rheological response of molten polyethylene was sensitive to both molecular weight distribution and shear modification. The effects of molecular weight distribution and shear modification were rationalized from a molecular point of view. A model is proposed which is based on the concept of a molecular network and incorporates polymer chain entanglement disruption and regeneration. Direct observations of polymeric semicrystalline morphologies in a copolyester by scanning electron microscopy were made possible by the development of a novel chemical etch. Spherulitic textures were consistent with classic spherulite growth mechanisms and structure theories. Uniaxial deformation of a single spherulite was successfully studied in a model system consisting of isolated spherulites embedded in an amorphous polymer matrix. By isolating the spherulite, the mechanical influence of surrounding and often impinging spherulites found in most semicrystalline polymers on the mechanical response of an individual spherulite was avoided. The mechanical response of the amorphous matrix was characterized and found to correlate with the effectiveness of a cold draw neck in elongating an embedded spherulite. The observed mechanism and morphology of isolated spherulite deformation were rationalized within the context of existing theories of spherulite-to-microfibrillar transitions. Optically active poly(L-lactic acid) and racemic poly(lactic acid) were synthesized in a ring opening polymerization scheme with stannous octoate as a catalyst and lactic acid as a molecular weight controlling initiator. Binary polymer blends composed of these isomeric polymer pairs were found to be miscible at 40,000 molecular weight and immiscible at 120,000 molecular weight. Strain hardening and the level of strain induced crystallization which occurred in the biaxial deformation of poly(lactic acid) blend films were found to be contingent on the concentration of optically active poly(L-lactic acid). Temperature, molecular weight, and biaxial strain rate were also found to have an influence on strain hardening and strain induced crystallization of these thermodynamically ideal polymer blends. / Ph. D.

High density polyethylene (HDPE) films having simple and well-defined stacked lamellar morphology, either with or without a distinct presence of row-nucleated fibril structures, have been utilized as <i>model</i> materials to carry out investigations on solid state structure-property relationships. Four different subjects that were addressed are: 1) mechanical properties and deformation morphologies, 2) orientation anisotropy of the dynamic mechanical α relaxation, 3) orientation dependence of creep behavior, and 4) crystalline lamellar thickness and its distribution. For the first three topics, appropriate mechanical tests, including tensile (INSTRON), creep (TMA), and dynamic mechanical (DMTA) tests, were performed at <i>different angles with respect to the original machine direction (MD)</i> of the melt extruded films; morphological changes as a result of these mechanical tests were detected by WAXS, SAXS, and TEM. For the forth topic, crystalline lamellar thickness and its distribution were determined by DSC, SAXS, TEM and AFM experiments. In the <i>large strain deformation</i> study (chapter 4.0), samples were stretched at 00°, 45° and 90° angles with respect to the original MD. A distinct orientation dependence of the tensile behavior was observed and <i>correlated</i> to the corresponding deformation modes and morphological changes, namely 1) lamellar separation and fragmentation by chain slip for the 00° stretch, 2) lamellar break-up via chain pull-out for the 90° stretch, and 3) lamellar shear, rotation and break-up through chain slip and/or tilt for the 45° stretch. A strong strengthening effect was observed for samples with row-nucleated fibril structures at the 00° stretch; whereas for the 90° stretch, the presence of such structures significantly limited deformability of the samples. In the <i>dynamic strain mechanical α relaxation</i> study (chapter 5.0), samples were tested at nine different angles with respect to the original MD, and the morphologies of samples <i>before</i> and </i>after</i> the dynamic tests were also investigated. The mechanical dispersions for the 00° and 90° tests were believed to arise essentially from the crystalline phase, and they contain contributions from two earlier recognized sub-relaxations of α_I and α_II. While for the 45° test, in addition to a high temperature α_II relaxation, a interlamellar shear induced low temperature mechanical relaxation was also observed. It is concluded that the low temperature relaxation is related to the characteristics of the interface between the crystalline lamellae and amorphous layers. In the <i>small strain creep</i> study (chapter 6.0), samples were tested at the 00°, 45° and 90° angles at the original MD. Both creep strain and creep rate for samples at the three angles were very different. An Eyring-rate model was utilized to analysis the observed creep behavior, and structural parameters associated with this model, including population of creep sites, activation energy and volume, were obtained by fitting the experimental data to the Eyring-rate equation. It was concluded that the plateau creep rate in these model materials is primarily controlled by the density and physical state of tie-chains in the amorphous phase. For the lamellar thickness and distribution study, DSC, SAXS, TEM and AFM experiments were conducted for samples having a well-defined stacked lamellar morphology. It was found that the most probable lamellar thickness from SAXS and TEM agreed very well; however, these values did not match with those obtained by DSC and AFM. It was pointed out that the use of DSC to determine lamellar thickness and distribution is so sensitive to heating rate and numerical values for the parameters in the Gibbs-Thomson equation that it is not believed to be suitable for quantitative analysis. / Ph. D.

While more than 100 years of research has focused on removing acute and chronic health threats from water, substantially less study has focused on potable water infrastructure and water quality deterioration, monitoring technologies, and relationships between water taste and consumer health. These knowledge–gaps have left infrastructure users, owners, regulators, and public health professionals largely unaware of how premise and buried polymer water pipes deteriorate and sorb/ desorb organic contaminants during normal operations and following water contamination events. These knowledge–gaps also prevent infrastructure managers from producing drinking water that optimizes mineral content for both water taste and health benefits, and employing a monitoring tool capable of immediately detecting water contamination or equipment failures. Research was conducted to address these challenges using analytical chemistry, environmental engineering, food science, polymer chemistry, public health, and material science principles. This work was enhanced by collaborations with sixteen American water utilities and the National Institute for Standards and Technology. These efforts were funded by the National Science Foundation, American Water Works Association, and the Water Research Foundation. Research results are unique and provide important scientific contributions to the public health, potable water, and material science industries. Particular achievements include the: (1) Evaluation of linkages between minerals, water palatability, and health useful for water production and public health decisions; (2) Creation of a novel infrastructure and water quality surveillance tool that has begun water utility implementation in the USA; (3) Development of an accelerated chlorinated water aging method with stable water pH, free chlorine, and alkalinity concentration that enables interpretation of polymer pipe surface and bulk characteristic changes; (4) Discovery that polar compounds are 2–193% more soluble in PEX than HDPE water pipes; (5) Finding that several polymer and contaminant properties can be used to predict contaminant diffusivity and solubility during sorption and desorption in new, lab aged, and water utility PE pipes; and the (6) Discovery that chlorinated water exposure of HDPE and PEX pipes increases polar contaminant diffusivity during sorption by 50–162% and decreases diffusivity during desorption as much as 211%. Outcomes of this work have domestic and global significance, and if engaged, can greatly improve public health protection, potable water infrastructure operations, water quality, sustainability, and regulation. / Ph. D.

This work is concerned with a better understanding of the influences that sparse long-chain branching has on the rheological and processing behavior of commercial metallocene polyethylene (mPE) resins. In order to clarify these influences, a series of six commercial polyethylenes was investigated. Four of these resins are mPE resins having varying degrees of long-chain branching and narrow molecular weight distribution. The remaining two resins are deemed controls and include a highly branched low-density polyethylene and a linear low-density polyethylene. Together, the effects of long-chain branching are considered with respect to the shear and extensional rheological properties, the melt fracture behavior, and the ability to accurately predict the flow through an abrupt 4:1 contraction geometry. The effects that sparse long-chain branching (M_branch > M_c) has on the shear and extensional rheological properties are analyzed in two separate treatments. The first focuses on the shear rheological properties of linear, sparsely branched, and highly branched PE systems. By employing a time-molecular weight superposition principle, the effects of molecular weight on the shear rheological properties are factored out. The results show that as little as 0.6 LCB/10⁴ carbons (<1 LCB/molecule) significantly increases the zero-shear viscosity, reduces the onset of shear-thinning behavior, and increases elasticity at low deformation rates when compared to linear materials of equivalent molecular weight. Conversely, a high degree of long-chain branching ultimately reduces the zero-shear viscosity. The second treatment focuses on the relationship between long-chain branching and extensional strain-hardening behavior. In this study, the McLeish-Larson molecular constitutive model is employed to relate long-chain branching to rheological behavior. The results show that extensional strain hardening arises from the presence of LCB in polyethylene resins, and that the frequency of branching in sparsely branched metallocene polyethylenes dictates the degree of strain hardening. This observation for the metallocene polyethylenes agrees well with the proposed mechanism for polymerization. The presence of long-chain branching profoundly alters the melt fracture behavior of commercial polyethylene resins. Results obtained from a sparsely branched metallocene polyethylene show that as few as one long-chain branch per two molecules was found to mitigate oscillatory slip-stick fracture often observed in linear polyethylenes. Furthermore, the presence and severity of gross melt fracture was found to increase with long-chain branching content. These indirect effects were correlated to an early onset of shear-thinning behavior and extensional strain hardening, respectively. Conversely, linear resins exhibiting a delayed onset of shear-thinning behavior and extensional strain softening were found to manifest pronounced slip-stick fracture and less severe gross melt fracture. The occurrence of surface melt fracture appeared to correlate best with the degree of shear thinning arising from both molecular weight distribution and long-chain branching. The ability to predict the flow behavior of long-chain branched and linear polyethylene resins was also investigated. Using the benchmark 4:1 planar contraction geometry, pressure profile measurements and predictions were obtained for a linear and branched polyethylene. Two sets of finite element method (FEM) predictions were obtained using a viscoelastic Phan-Thien/Tanner (PTT) model and an inelastic Generalized Newtonian Fluid (GNF) model. The results show that the predicted profiles for the linear PE resin were consistently more accurate than those of the branched PE resin, all of which were within 15% of the measured vales. Furthermore, the differences in the predictions provided by the two constitutive models was found to vary by less than 5% over the range of numerical simulations obtained. In the case of the branched PE resin, this range was very narrow due to loss of convergence. It was determined that the small differences between the PTT and GNF predictions were the result of the small contraction ratio utilized and the long relaxation behavior of the branched PE resin, which obscured the influence of extensional strain hardening on the pressure predictions. Hence, it was expected that numerical simulations of the 4:1 planar contraction flow for the mildly strain hardening metallocene polyethylenes would not be fruitful. / Ph. D.

Cellulose nanocrystals (CNCs) are nanomaterials that have shown promise as reinforcement filler materials. Their small size, high modulus, and high aspect ratio makes CNCs good reinforcing materials. CNCs are typically introduced into softer polymer materials, which can have incompatible surface chemistry such as aliphatic chains, leading to aggregation and poor reinforcement of the material. The intrinsic hydrophobicity of the CNC surfaces suggests that dispersal into hydrophobic polymer matrices, which the CNCs could potentially reinforce, represent a significant challenge. Therefore, new non-traditional strategies are needed to introduce CNCs into polymer materials. The hydroxyl groups on the surfaces of CNCs can be functionalized using a variety of chemical techniques to yield materials that can interact better with solvents or polymers. Additionally, surface groups can allow the CNCs to react with environmental stimuli (smart materials). The primary focus of this work is the incorporation of CNCs in hydrophobic matrices. Herein we introduce a new method of dispersing CNCs in polyethylene (PE), a substance of legendary hydrophobicity that is also the most common synthetic polymer used in consumer packaging. The prospect of increasing the mechanical strength of PE by incorporating CNC materials as fillers may lead to the possibility of using less polymer to obtain the same strength. This thesis approaches the problem of dispersing CNCs within PE by first functionalizing the CNCs with a catalyst capable of polymerizing ethylene and other α-olefins. The catalyst 1,1'-bis(bromodimethylsilyl)zirconocene dibromide (catalyst 1) is equipped with anchoring groups that are capable of attachment to the surface hydroxyl groups of CNC particles. After immobilizing catalyst 1 onto various CNC samples, introduction of solvent, organoaluminum cocatalyst, and monomer (ethylene alone or ethylene plus 1-hexene) afforded high density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) samples, respectively, containing well-dispersed CNCs as filler materials. Chapter 2 provided important information on the attachment of catalyst 1 to cellulose nanocrystals and the successful polymerization of ethylene from the cellulose nanocrystals. The resulting composite materials showed a in Young's modulus that was three-fold that of PE samples we tested (1600 ± 100 vs 500 ± 30) and about 10% greater relative to a commercial high modulus PE sample (1450 MPa). The increase in Young's modulus along with the lack of macroscopic aggregates led to the conclusion that we have developed a viable method to disperse CNCs in polyolefin matrices. Chapter 3 focused on the dispersal of CNCs in a softer, more pliable polyethylene grade known as linear low-density polyethylene (LLDPE). LLDPE incorporates a small fraction of 1-hexene into polyethylene as a randomly inserted comonomer, giving rise to properties suitable for applications in plastic films and bags among other end uses. Catalyst 1 functionalized CNCs were added to a reaction vessel with both ethylene and 1-hexene to afford LLDPE CNC composites. Different loading of catalyst 1 on CNC aerogels afforded the same amount of catalyst in each reaction but allowed for different CNC loadings in each reaction. The composite materials showed increasing Young's modulus with increasing cellulose nanocrystal content. Chapter 4 describes how CNCs were functionalized with the intention of filling reverse osmosis membrane materials to have surface chemistry that could be impart antibacterial properties and increase flux. CNCs were functionalized with carboxylic acid by 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-mediated oxidation, then amine functionalization by carbodiimide coupling chemistry, and finally functionalized with a zwitterionic group by β-propiolactone ring opening. Amine coupling was confirmed with X-ray photoelectron spectroscopic analysis, and a second carboxylic acid peak was confirmed using infrared spectroscopy. These results were further verified with conductometric titration showing that after each respective reaction there were 1060 mmol kg-1 of carboxylic acid groups, 520 mmol kg-1 of amine groups, and 240 mmol kg-1 of zwitterionic groups. This CNC material was left to undergo future testing for desirable membrane properties. Chapter 5 assesses the possible value in creating a new composite material using a functionalized polynorbornene, poly(5-triethoxysilyl-2-norbornene) (PTESN). The composites were fabricated by using the solvent casting method, dispersing the CNCs in a toluene solution of polymer and drying. The composite materials showed an increase in Young's modulus with increased loading. The 20 wt% CNC in PTESN had a Young's modulus of 970 MPa, a significant increase over the Young's modulus of the polymer lacking the filler (540 MPa). In summary, this dissertation advances new techniques for the incorporation of CNCs as fillers in polymer-based nanocomposites. We are confident that further refinement and development of our results will find wide-ranging application. / Doctor of Philosophy / Cellulose nanocrystals (CNCs) are materials that can be added to polymers to form composite materials having increased stiffness. CNCs have the primary advantages over other filler materials of providing significant reinforcement without changing the color or increasing the density of the overall composite. CNCs are therefore good for designing polymer composites that need to be lightweight and aesthetically pleasing. Packaging materials (especially plastic bags and plastic films) are dominated by polyolefin materials such as polyethylene, which is already lightweight and colorless. The challenge of mixing polyethylene and CNCs is that their surface chemistry is incompatible, "like oil and water." To overcome the natural tendency for the CNC filler material to separate from the surrounding polyethylene matrix, a catalyst was attached to the surface of the CNCs and polymerization ensued from that catalyst leading to a composite material in which tiny CNC particles were trapped in the matrix Good dispersal of the component substances in the composite and of excellent overall reinforcement were proven by physical analysis.

With this research, we work towards the overall objective of customizing polymer molecules in terms of their molecular structure to optimize processing performance. The work includes analysis of the rheology in shear and shear-free flows for sparsely long-chain branched, LCB, polyethylene, PE, resins; determination of the consistency of the molecular based constitutive model, the pom-pom model; for these flows, and evaluation of the same PE resins in film-casting. As we progress towards molecular systems with defined molecular structural characteristics, we transition from a linear low density polyethylene, LLDPE, based series of PE resins to a high density polyethylene, HDPE, based series of PE resins, each with materials of varying degrees of sparse LCB. Evaluation of the shear step-strain rheology for the series of LLDPE-based PE resins allows for the assessment of any inadequacies associated with the step-strain experiments and the ability of the K-BKZ analog of the pom-pom constitutive model to predict step-strain rheological behavior. Finite rise time and wall slip are addressed to ensure the accuracy of the experimental step-strain measurements and eliminated as factors contributing to the stress relaxation moduli response. Analysis of the K-BKZ analog of the pom-pom constitutive model includes comparisons between experimental stress relaxation moduli and predictions from the model using pom-pom model parameters determined from extensional rheology. The results show inconsistencies in the model predictions, where the predictions fail to capture the short time behavior and accurately dampen at larger strains. Pom-pom model parameters are determined using the K-BKZ analog of the pom-pom constitutive model and fitting the stress relaxation moduli. These results are qualitatively consistent indicating that branching occurs on the longest backbone segments, but the values appear to be unrealistic with respect to the molecular theory. Analysis of film-width reduction or necking during film-casting for the series of LLDPE-based resins determines whether uniaxial extensional rheological characteristics, in particular strain-hardening, that are a result of LCB influence the film-necking properties. At the lowest drawdown ratio necking is observed to be reduced with increasing LCB, and thus strain-hardening characteristics. At the higher drawdown ratios it is observed that LCB no longer reduces necking and the curves merge to the results found for linear PE, except in the case of LDPE, which shows reduced necking at all drawdown ratios. Furthermore, comparisons of film necking are also made to separate the effects of molecular weight distribution, MWD, and LCB. The results indicate that both broadening the MWD and the addition of sparse LCB reduce the degree of necking observed. It is established that film necking is more significantly reduced by LCB than by broadening the MWD. Analysis of the uniaxial extensional and dynamic shear rheology with the pom-pom constitutive model reveals that a distribution of branches along shorter relaxation time modes is important in reducing necking at higher drawdown ratios. Factors such as shear viscosity effects, extrudate swell, and non-isothermal behavior were eliminated as contributing factors because of the similar shear viscosity curves, N1 curves, and activation energies among the sparsely LCB PE resins. The same experimental concepts have been extended to the series of HDPE-based resins, but the lack of adequate uniaxial extensional data prevents a thorough analysis with respect to uniaxial extensional characteristics. Regardless, in the context of step-strain rheology, the results were found to be similar with those of the LLDPE-based series of resins, where a distinctive shape at short times was observed for any of the PE resins possessing some level of LCB that was not apparent in the linear PE resins. Film-casting revealed similar results to those of the LLDPE-based materials as well, but a broader spectrum of drawdown ratios revealed greater insight into how the distribution of branching controls the film-casting response. At low drawdown ratios all materials exhibit the same necking behavior. At intermediate drawdown ratios separation occurs where the linear PE resins experiences the most drastic necking, the sparsely LCB PE resins show reduced necking, and the LDPE shows an even greater reduction in necking. Progression then to the higher drawdown ratios results in similar necking behavior for the linear and sparsely LCB PE resins and greatly reduced necking for the LDPE. These results support the idea that to reduce necking the backbone segments that dominate the film-casting behavior must contain some level of LCB. / Ph. D.