For the purpose of studying morphologies of ultra-thin adsorbed layers, novel polystyrenepoly(2-~inylpyridine)-polybutadiene (PS-PVP-PB) three-block polymers were synthesized with a multistep coupling method. First, end-chlorinated PS-PVP-Cl diblock precursors were made via termination of living PS-PVP-anions with a large excess of p-xylylene dichloride (XDC). Those PS-PVP-C1 precursors were precipitated in heptane under vacuum to reduce substantially the amount of unreacted XDC left with them and then were allowed to be coupled with PB precursors having diphenylethylene (DPE) anion ends. The desired 1:l coupling of the PS-PVP-C1 and PB-DPE-precursors was successfully achieved, and the PS-PVP-PB three-block polymers were obtained after fractionation. Preliminary surface measurements were carried out for ultra-thin, adsorbed layers of some of those PS-PVP-PB polymers. Those layers were prepared on mica, and were examined in their as-dried state without any heat treatment. X-ray photoelectron spectroscopy and contact angle measurement9 revealed that the PVP blocks were located at the bottom of the layers while the PS and PB blocks were at the top. Surface profiles of the top layers were examined with an atomic force microscope (AFM), and nodules of height = 1 nm and width z 20-50 nm were observed. Similar nodules were found for an adsorbed layer of a PS-PVP diblock polymer, but not for a PB-PVP layer. This result suggested that the nodules for the PS-PVP-PB layers were the domains of the PS blocks. Mechanisms of the nodule formation as well as the location of the PB blocks in the PS-PVP-PB adsorbed layers are discussed.

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High-density polyethylene (HDPE) pipe and piping components have been used successfully and safely for natural gas distribution around the world for several decades. The primary concerns for a 50-year life for buried HDPE piping involves designing against three primary failure modes—ductile fracture, rapid crack propagation (RCP), and slow crack growth (SCG) under sustained pressure loading. Although, design methodologies for preventing ductile fracture and RCP are well established, SCG remains to be a limiting failure mode in determining useful service life of HDPE piping as it may occur under sustained pressure and temperature. Although considerable amount of research has been conducted over the last two decades, SCG still remains less well understood than other failure modes. A critical evaluation of various test methodologies available to determine the SCG resistance of HDPE resins was conducted using finite element analysis (FEA) of various widely used laboratory test specimens. While there exist extensive information on the test methodologies and the applicability of each of the SCG testing methods, there is a growing concern as to whether any/all of these SCG tests give the same information akin to the industrial pipe application, particularly so when conflicting messages are obtained from time to failure predictions from two different SCG tests. While notched-pipe test (NPT) proves to be a direct approach to assess SCG resistance of the polyethylene (PE) pipe with the use of temperature as a test accelerating factor; in the case of newer grade PE resins, the failure time of NPT can still be considerably large (∼5000 to 10,000 h). For this reason, some of the other coupon SCG tests are focus of recent investigations and especially sought after for rapid ranking/assessment of resins and understanding the manufactured HDPE pipe performance. In this study, FEA was conducted to facilitate a direct comparison of leading SCG test methods, through determination of both the stress intensity factor, KI, and existing constraint factors in various widely used specimen geometries. These results are then compared to pipe specimen with an outer diameter (OD) or inner diameter (ID) surface notch. Since, constraint can have a significant role in SCG initiation, transverse/constraint stress (T-stress), and biaxiality ratios (β), these were compared along the crack fronts to arrive at definitive reasons for the smaller failure times observed when testing some of the SCG test specimens, and also reasons for SCG mode of failure observed even under large applied loads (large KI compared to that in a notched pipe) when testing some of the SCG test specimens. The use of stress intensity factor, KI, along with the T-stress and biaxiality ratio (β), was found to provide a complete picture on the broad spectrum of failure times observed from various SCG test specimens, and rationale for choosing a SCG test specimen when evaluating HDPE pipe or resins.

HDPE pipe and piping components have been used successfully and safely for natural gas distribution around the world for several decades. The primary concerns for a 50-year life for buried HDPE piping involves designing against three primary failure modes — ductile fracture, rapid crack propagation (RCP), and slow crack growth (SCG) under sustained pressure loading. Although design methodologies for preventing ductile fracture, and RCP are well established, SCG remains to be a limiting failure mode in determining useful service life of HDPE piping as it may occur under sustained pressure and temperature. Although considerable amount of research has been conducted over the last two decades, SCG still remains less well understood than other failure modes. A critical evaluation of various test methodologies available to determine the SCG resistance of HDPE resins was conducted using FEA of various widely used laboratory test specimens. While there exist extensive information on the test methodologies and the applicability of each of the SCG testing methods, there is a growing concern as to whether any/all of these SCG tests give the same information akin to the industrial pipe application, particularly so when conflicting messages are obtained from time to failure predictions from two different SCG tests. While notched-pipe test (NPT) proves to be a direct approach to assess SCG resistance of the PE pipe with the use of temperature as a test accelerating factor; in the case of newer grade PE resins, the failure time of NPT can still be considerably large (∼5,000 to 10,000 hours). For this reason, some of the other coupon SCG tests are focus of recent investigations and especially sought after for rapid ranking/assessment of resins and understanding the manufactured HDPE pipe performance. In this study, FEA was conducted to facilitate a direct comparison of leading SCG test methods, through determination of both the stress intensity factor, KI, and existing constraint factors in various widely used specimen geometries. These results are then compared to pipe specimen with an OD (outer diameter) or ID (inner diameter) surface notch. Since, constraint can have a significant role in SCG initiation, T-stress, and biaxiality ratios (β), these were compared along the crack fronts to arrive at definitive reasons for the smaller failure times observed when testing some of the SCG test specimens, and also reasons for SCG mode of failure observed even under large applied loads (large KI compared to that in a notched pipe) when testing some of the SCG test specimens. The use of stress intensity factor, KI, along with the T-stress and biaxiality ratio (β), is found to provide a complete picture on the broad spectrum of failure times observed from various SCG test specimens and rationale for choosing a SCG test specimen when evaluating HDPE pipe or resins.

No abstract