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Electronic Resource

Evidence for non-site-isolation in ZSM-5 from ion-exchange and catalytic studies (1991)

Dijkstra, P. C. ; Nicolaides, C. P. ; Weiss, K. I. G.

Springer

Catalysis letters 10 (1991), S. 375-382

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ISSN: 1572-879X

Keywords: ZSM-5 ; ion-exchange ; alkali metal cations ; n-hexane cracking ; catalytic test reaction

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology

Notes: Abstract Ion exchange of HZSM-5 samples with alkali metal cations, using metal chloride solutions, results in partially exchanged zeolites, MHZSM-5, M = Li, Na, K or Cs. The degree of exchange is found to increase with increasing ionic radius of the cations. The catalytic properties of the alkalized zeolites were evaluated using the reaction conditions under which the catalytic activity of the HZSM-5 samples in terms of n-hexane cracking is proportional to the aluminium content. From the residual catalytic activity exhibited by the Na-, K- and CsHZSM-5 samples it is concluded that each of the larger Na+, K+ and Cs+ ions is influencing more than one AlO 4 − tetrahedron, implying that the aluminium sites in ZSM-5 are not isolated. The ion-exchange results are then interpreted in terms of non-isolated aluminium sites. The ion-exchange and catalytic properties of the zeolites as a function of aluminium content are also discussed.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00769172

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2

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Photosynthetic rates in relation to leaf phosphorus content in pioneer versus climax tropical rainforest trees (1995)

Raaimakers, D. ; Boot, R. G. A. ; Dijkstra, P. ; [et al.]

Springer

Oecologia 102 (1995), S. 120-125

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ISSN: 1432-1939

Keywords: Climax tree species ; Phosphorus Photosynthesis ; Pioneer tree species ; Tropical rainforest

Source: Springer Online Journal Archives 1860-2000

Topics: Biology

Notes: Abstract In Guyana dense rainforest occurs on intensely weathered acid soils, low in soil phosphorus. To investigate whether low P availability limits photosynthesis of trees growing on these soils more than N does, leaf P and N content, and their relationship with the photosynthetic capacity (A sat, μmol CO2 m-2 s-1) were studied for nine pioneer and climax tree species in a range of light climates. Light environment was described using hemispherical photographs. For both pioneer and climax species, leaf P content (r 2=0.71 and 0.23, respectively) is a more important determinant of A sat than leaf N content (r 2=0.54 and 0.12, respectively). Pioneer species have a higher leaf P and N content than climax species. At similar P or N content, pioneers have a higher A sat than climax species. The saplings studied had a relatively high A sat, considering their low P concentration (15–30 μmol P g-1). All species studied had a constant leaf P and N concentration and photosynthetic capacity across light climates, because specific leaf mass (g m-2) increased similarly with light availability. This acclimation to a change in light environment makes a possible limitation of A sat by P or N independent of light environment.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00333319

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3

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Glutaraldehyde as a crosslinking agent for collagen-based biomaterials (1995)

Olde Damink, L. H. H. ; Dijkstra, P. J. ; Luyn, M. J. A. ; [et al.]

Springer

Journal of materials science 6 (1995), S. 460-472

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ISSN: 1573-4838

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: The formation of Schiff bases during crosslinking of dermal sheep collagen (DSC) with glutaraldehyde (GA), their stability and their reactivity towards GA was studied. All available free amine groups had reacted with GA to form a Schiff base within 5 min after the start of the reaction under the conditions studied (0.5% (w/w) GA). Before crosslinks are formed the hydrolysable Schiff bases initially present were stabilized by further reaction with GA molecules. An increase in shrinkage temperature (T s) from 56°C for non-crosslinked DSC (N-DSC) to 78°C for GA crosslinked DSC (G-DSC) was achieved after crosslinking for 1 h. From the relationship between the free amine group content and the T s during crosslinking it was concluded that higher GA concentrations and longer reaction times will result in the introduction of pendant-GA-related molecules rather than crosslinks. After 24 h crosslinking an average uptake of 3 GA molecules per reacted amine group was found. No increase in the tensile strength of the materials was observed after crosslinking, which may be a result of formation of crosslinks within the fibres rather than in between fibres. Aligning of the fibres by applying a pre-strain to the samples and subsequent crosslinking yielded materials with an increased tensile strength.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00123371

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4

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Crosslinking of dermal sheep collagen using hexamethylene diisocyanate (1995)

Olde Damink, L. H. H. ; Dijkstra, P. J. ; Luyn, M. J. A. ; [et al.]

Springer

Journal of materials science 6 (1995), S. 429-434

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ISSN: 1573-4838

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: The use of hexamethylene diisocyanate (HMDIC) as a crosslinking agent for dermal sheep collagen (DSC) was studied. Because HMDIC is only slightly water soluble, a surfactant was used to obtain a clear and micellar crosslinking solution and to promote the penetration of HMDIC in the DSC matrix. Using optimized conditions treatment of non-crosslinked DSC (N-DSC) with HMDIC (H-DSC) increased the shrinkage temperature (T s) of N-DSC from 56°C to 74°C for H-DSC. A linear relation between the decrease in free amine group content and the increase in T s was observed. Crosslinking with HMDIC did not influence the tensile strength of the N-DSC samples but increased the elongation at break from 141% to 163% and decreased the high-strain modulus from 26 MPa to 16 MPa for the H-DSC samples, respectively.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00120286

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5

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Calcification of subcutaneously implanted collagens in relation to cytotoxicity, cellular interactions and crosslinking (1995)

Luyn, M. J. A. ; Wachem, P. B. ; Dijkstra, P. J. ; [et al.]

Springer

Journal of materials science 6 (1995), S. 288-296

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ISSN: 1573-4838

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: In general, calcification of biomaterials occurs through an interaction of host and implanted material factors, but up to now the real origin of pathologic calcification is unknown. In this study we aimed to investigate incidence of calcification of (crosslinked) dermal sheep collagens (DSCs) with respect to their specific properties, during subcutaneous implantation in rats. Three types of DSCs were commercially obtained: non-crosslinked DSC (NDSC), and DSC crosslinked with glutaraldehyde (GDSC) and hexamethylenediisocyanate (HDSC). NDSC, HDSC and GDSC were (enzymatically) tissue culture pretreated to eliminate their cytotoxic products. Beside this, crosslinking methods were modified to optimize mechanical properties and to decrease cytotoxicity, which resulted in HDSC* and GDSC*. Furthermore, DSC was crosslinked by activation of the carboxylic groups, i.e. by means of acyl azide and carbodiimide, resulting in AaDSC and CDSC, respectively. After implantation of HDSCs and GDSCs a relation between cytotoxicity and calcification of crosslinked DSC could be made. No relation was found between cellular infiltration of DSCs and calcification. However, from the use of different types and modification of crosslinking methods it might be concluded that calcification is mainly related to stable crosslinks, i.e. to the chemical properties of the obtained material.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00120273

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6

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Biodegradation of porous versus non-porous poly(L-lactic acid) films (1994)

Lam, K. H. ; Nieuwenhuis, P. ; Molenaar, I. ; [et al.]

Springer

Journal of materials science 5 (1994), S. 181-189

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ISSN: 1573-4838

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: The influence of porosity on the degradation rate of poly(L-lactic acid) (PLLA) films was investigated in vitro and in vivo. Non-porous, porous and “combi” (porous with a non-porous layer) PLLA films were used. Changes in Mw, Mn, polydispersity (Mw/Mn) ratio, melting temperature (T m), heat of fusion, tensile strength, E-modulus, mass and the remaining surface area of cross-sections of the PLLA films were measured. In general, during the degradation process, the porous film has the highest Mw, Mn, Mw/Mn ratio and T m, while the non-porous film has the lowest. In contrast, the highest heat of fusion values were observed for the non-porous film, indicating the presence of relatively smaller molecules forming crystalline domains more easily. The tensile strength and E-modulus of the non-porous film decrease faster than those of the porous and the combi film. None of the three types of films showed massive mass loss in vitro nor a significant decrease in remaining polymer surface area in light microscopical sections in vitro and in vivo. Heavy surface erosion of the non-porous layer of the combi film was observed after 180 days, turning the combi film into a porous film. This is also indicated by the changes in tensile strength, Mw, Mw/Mn, T m and heat of fusion as a function of time. It is concluded that non-porous PLLA degrades faster than porous PLLA. Thus, in our model, porosity is an important determinant of the degradation rate of PLLA films.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00121086

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