Deformation and failure of polymer glasses

This thesis is composed of six papers in the field of deformation and failure of polymer glasses. Prediction of deformation and failure behaviour of polymers has become very important. In the last two decades considerable effort is addressed to the development of 3D constitutive models that were able to capture the visco-elastic and post-yield behaviour of glassy polymers. The compressible Leonov model, as developed in our group, proved to be a suitable model which provides an adequate description of this behaviour, including rate- and temperature-dependent yield, strain softening and strain hardening. However a failure criterion is still lacking. Previous studies indicated that macroscopic deformation behaviour is dominated by the intrinsic post-yield behaviour. Improving the ductility should hence focus on avoiding! r localisation of strain by elimating strain softening and promoting the contribution of the strain hardening. Although it is quite well established that strain hardening originates from the contribution of the entangled polymer network, the high strain hardening modulus compared to the rubber-modulus and its temperature dependence requires further investigation. The physical origin of strain softening is less well known, although it is reported that strain softening can be altered by thermal treatments and can even be eliminated by mechanical rejuvenation. The limited resistance to void nucleation and the build-up of high dilative stresses under certain loading conditions, show that decreasing strain softening and increasing strain hardening are not sufficient to achieve tough deformation behaviour. To circumvent these problems heterogeneity should be introduced in the structure to relieve the build-up of high hydrostatic stresses. For materia! ls like polycarbonate and polyamide this results in a transition from crazing to shear yielding. For polystyrene this is only the case if the thickness of the ligaments within the structure are sufficiently small. The concept of a critical thickness suggests that an absolute length-scale is encountered. An absolute length-scale of the same order of magnitude as is found in mechanical tests, is also reported in polymer physics where a Tg-depression is found in thin polystyrene films. In chapter 2 the influence of the network density on the strain hardening modulus is investigated. An increase in network density of polystyrene, achieved by cross-linking and blending with polyphenylene oxide results in a proportional increase in strain hardening modulus. It is discussed that the maginitude of the strain hardening modulus and its negative temperature dependence might orginate from the time-scale of the stress-induced segmental mobility and tha! t, on this time-scale, the secondary interactions still play a significant role. The transient deformation behaviour of mechanically rejuvenated polstyrene in studied in chapter 3. Although the recovery of yield stress and strain softening is independent of the molecular weight, the time to re-embrittlement proves to increase with increasing molecular weight. This is rationalised by the fact that the tensile strength of the material, and hence the recovered yield stress at which this strength is exceeded in a localised plastic zone, depends on the molecular weight. The post-yield behaviour dominates the macroscopic deformation behaviour of amorphous polymers. In chapter 4 it is shown that polycarbonate with its moderate strain softening and strong strain hardening results is stable neck growth during deformation. By annealing the strain softening increases, leading to more severe localisation of strain and even brittle failure. The deformatio! nmode can be be predicted in a straight-forward manner using a stability analysis. The pronounced strain softening and weak strain hardening of polystyrene lead to extreme localisation of strain and explain that standard polystyrene can never be ductile. Elimination of strain softening by mechanical rejuvenation inhibits localisation of strain and results in (temporary) ductile deformation behaviour. Additional finite element simulations illustrate the route to improve ductility. Since a failure criterion was still lacking in the finite element simulations employing the compressible Leonov model, micro-indentation experiments are used to generate crazes in a reproducible way. By evaluation of the local stress and strain distribution by finite element simulations, a critical hydrostatic stress of 40 MPa was found in polystyrene (provided that this event is preceded by plastic deformation) as a criterion for void nucleation. This criterion prov! ed to be independent of thermal history and strain rate but proved to increase with network density. By means of micro- and nano-indenations on polystyrene the influence of an absolute length-scale, as reported in other areas of polymer science, is investigated. For large indenters and indentation depth the experiments compare well to the length-scale independent finite element simulations, using bulk properties. For the smallest indenter (2.2 ??m) and shallow indentation depth (100 nm) the resistance to indentation is much less than expected from the simulations, indicating that the mechanical properties near a free surface in polystyrene might differ from the bulk properties. Using the criterion for void nucleation, as identified in chapter 5, brittle-to-ductile transitions (BDTs) were predicted by the deformation of a representative volume element (RVE). By increasing the temperature in the RVE, the overall stress level lowers in such a ! way that at 70ÆC the critical level of 40 MPa is not exceeded anymore in the simulations and hence a transition from crazing to shear yielding is achieved in polystyrene. The length-scale which is encountered experimentally and numerically in chapter 6 was incorporated in the RVE by assuming a gradient of increased temperature near free surfaces. At an interparicle distance of less than approximately 15 nm the critical value of 40 MPa is not exceeded anymore and crazing is hence inhibited. Both brittle-to-ductile transition compare well to experimental observations

Evaluation of minimally invasive surgical techniques in clinical BPH

Introduction and objectives: There are more and more men who suffer from lower urinary tract symptoms (LUTS) associated with benign prostatic hyperplasia (BPH). Symptoms can be classified as storage symptoms or voiding symptoms, e.g. weak urinary flow and urgency-frequency. Several treatment options are possible. Transurethral resection of the prostate is the gold standard for surgical therapy. There is a correlation between prostatic obstruction and these symptoms. However, there is some evidence that patients who are not obstructed benefit from surgery. Two studies presented in this thesis compare obstructed with unobstructed patients who underwent TURP because of LUTS and associated BPH. TURP shows several complications like bleeding. New surgical modalities have been introduced to match the results of TURP, but with less complications. Several studies described in this thesis compare the results of TURP with contact laser prostatectomy (CLP) and electrovaporization (Evap). Costs of these treatments are nowadays of great importance and are analyzed in a separate chapter. A special group of patients with LUTS and BPH who suffer from bleeding disorders or use anticoagulants is treated with laser therapy. Materials and methods: 132 patients were urodynamically measured before and 6 months after TURP. They also completed validated questionnaires to quantify symptoms and subjective results. Urodynamic outcome was compared to subjective outcome. A subgroup af patients with a Schaefer grade of 1 or 2 (unobstructed or equivocal) were compared to patients who were obstructed according to Schaefer. A randomized, controlled study was conducted comparing TURP, CLP and Evap. Fifty, 45 and 46 patients were included respectively. Preoperatively and postoperatively up to 7 years tests were performed, including urodynamics, uroflowmetry, validated symptom questionnaires, morbidity and mortality. A separate cost analysis was performed. A prospective, controlled study of high risk patients was conducted, in which 30 patients with bleeding disorders or using anticoagulants were treated with laser therapy. They were compared to 45 normal risk patients undergoing laser therapy. Results and conclusions: Patients with a high obstruction grade benefited most from TURP, but those who were not obstructed showed also clinical relevant improvements in objective and subjective parameters. Urodynamic, uroflowmetric, symptomatic and subjective results, morbidity and mortality were very similar for all three groups at several times during the first year postoperatively and during long-term follow-up up to 7 years (mean follow-up 4.3 years). Costs were lowest for Evap and almost similar for CLP and TURP. Laser therapy is a safe and effective operation for high-risk patient

The Dutch language short Anterior Cruciate Ligament—Return to Sport after Injury scale has good to excellent construct validity, internal consistency, and test-retest reliability when assessing athletes undergoing rehabilitation after ACL injury or ACL reconstruction

Objectives: To examine the validity and reliability of the Dutch language short Anterior Cruciate Ligament—Return to Sport after Injury scale (short ACL-RSI-NL) in recreational athletes undergoing rehabilitation after ACL injury or ACL reconstruction (ACLR). Methods: The original 12-item version of the ACL-RSI had been translated into Dutch. Short ACL-RSI-NL items were derived from this 12-item Dutch version. Content validity was evaluated by a team consisting of eight ACL experts and eight athletes. A cohort of 115 athletes with ACL injury or after ACLR completed the short ACL-RSI-NL and related questionnaires at various time points during their rehabilitation. Construct validity (hypothesis testing using Spearman correlations), internal consistency (Cronbach's alpha), floor and ceiling effects (percentage of athletes having the lowest or highest score possible), and structural validity (exploratory factor analysis) were evaluated in the entire ACL athlete group. Test-retest reliability (using intra-class correlation, ICC; standard error of measurement, SEM; smallest detectable change, SDC, at both group and individual levels) was investigated in a subgroup of athletes with a stable outcome on psychological readiness within a two-week interval (n ​= ​27). Results: The short ACL-RSI-NL demonstrated good construct validity (83% of hypotheses confirmed). Internal consistency was excellent (Cronbach's alpha 0.84), and there were no floor and ceiling effects (≤13.9% lowest or highest score). Test-retest reliability was good (ICC 0.89 with 95% CI 0.77–0.95, SEM 6.93, SDC individual level 19.2, SDC group level 3.7). Exploratory factor analysis confirmed the presence of a single underlying factor (accounting for 56.4% of the total variance of the score). Conclusion: The short ACL-RSI-NL exhibited good to excellent construct validity, internal consistency, and test-retest reliability. An averaged score ranging from 0 to 100 can be used to measure psychological readiness to return to sport. The short ACL-RSI-NL has potential for use in day-to-day practice to assess the psychological readiness of recreational athletes to return to sport after ACL injury or ACLR during their rehabilitation process. Level of evidence: Level II.</p

Scaling of the Strain Hardening Modulus of Glassy Polymers with the Flow Stress

In a recent letter, Govaert et al. examined the relationship between strain hardening modulus $G_r$ and flow stress $\sigma_{flow}$ for five different glassy polymers. In each case, results for $G_r$ at different strain rates or different temperatures were linearly related to the flow stress. They suggested that this linear relation was inconsistent with simulations. Data from previous publications and new results are presented to show that simulations also yield a linear relation between modulus and flow stress. Possible explanations for the change in the ratio of modulus to flow stress with temperature and strain rate are discussed.Comment: 8 pages, 2 figures: clarified arguments on linear proportionality. Accepted for publication in J. Poly. Sci Part B - Polym. Phy

High etch rate and smooth morphology using a novel chemistry in reactive ion etching of GaN

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Robust process windows for Laser Induced Forward Transfer of thin film metal to create inter connects

Direct-write technologies can form a low-cost, alternative approach to create electrical interconnects by eliminating mask and etch costs. Also, direct-write is more efficient in creating complex structures as well as for producing small series. However, existing, industrially-mature direct-write technologies typically lack the resolution required for advanced IC packaging applications [1-4]. Laser Induced Forward Transfer (LIFT) is a direct write process which has been proven to be capable of patterning resolutions in the 1-5 μm range [5-8]. Thus far, a lack of deposition control resulting in contamination of the substrate has been a problem. The current paper shows an approach to come to a robust, contamination-free process window for LIFT of pure copper. Thus, we tackled a major roadblock towards the industrial feasibility of LIFT as a full metal direct-write technology that meets the current demands for IC packaging and integration

Stress overshoot in a simple yield stress fluid: an extensive study combining rheology and velocimetry

We report a large amount of experimental data on the stress overshoot phenomenon which takes place during start-up shear flows in a simple yield stress fluid, namely a carbopol microgel. A combination of classical rheological measurements and ultrasonic velocimetry makes it possible to get physical insights on the transient dynamics of both the stress $\sigma(t)$ and the velocity field across the gap of a rough cylindrical Couette cell during the start-up of shear under an applied shear rate $\dot\gamma$. (i) At small strains ($\gamma <1$), $\sigma(t)$ increases linearly and the microgel undergoes homogeneous deformation. (ii) At a time $t_m$, the stress reaches a maximum value $\sigma_m$ which corresponds to the failure of the microgel and to the nucleation of a thin lubrication layer at the moving wall. (iii) The microgel then experiences a strong elastic recoil and enters a regime of total wall slip while the stress slowly decreases. (iv) Total wall slip gives way to a transient shear-banding phenomenon, which occurs on timescales much longer than that of the stress overshoot and has been described elsewhere [Divoux \textit{et al., Phys. Rev. Lett.}, 2010, \textbf{104}, 208301]. This whole sequence is very robust to concentration changes in the explored range ($0.5 \le C \le 3$ w/w). We further demonstrate that the maximum stress $\sigma_m$ and the corresponding strain $\gamma_m=\dot\gamma t_m$ both depend on the applied shear rate $\dot \gamma$ and on the waiting time $t_w$ between preshear and shear start-up: they remain roughly constant as long as $\dot\gamma$ is smaller than some critical shear rate $\dot\gamma_w\sim 1/t_w$ and they increase as weak power laws of $\dot \gamma$ for $\dot\gamma> \dot\gamma_w$ [...].Comment: 18 pages, 14 figures, accepted for publication in Soft Matte