פולימרים

Discuss, Learn and be Happy דיון בשאלות

• «
• 1
• 2
• 3
• 4
• 5
• 6
• »

Free radical polymerization of some vinyl polymers can give polymer chains with syndiotactic placement of monomers in the chain, under which of the following conditions, (Choose from the following four possibilities)

-

1

+

שאלה #38975

		if the free radical growing chain end has a cationic counter ion.
		If the initiation step is done at high temperatures.
		If the vinyl monomer has a large group on both carbons (e.g., CR'H=CHR) of the vinyl group and if the free radical polymerization is carried out at low temperature.
done		If the vinyl monomer has a relatively large head group R on one head group (e.g., CH2=CHR) and if the free radical polymerization is carried out at low temperature.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Group transfer polymerization is similar to which of the following polymerization processes with respect to the ratio of number average molecular weight to weight average molecular weight (Mn/ Mw) it can achieve?

-

1

+

שאלה #38976

		Free radical polymerization with benzoyl peroxide.
done		Anionic polymerization without chain termination.
		Cationic polymerization with BF3/H2O
		Friedel-Crafts polymerization used in condensation polymerization reactions

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Group transfer polymerization is similar to which of the following polymerization processes with respect to the MW homogeneity it can achieve?

-

1

+

שאלה #38977

		Free radical polymerization with benzoyl peroxide
		Cationic polymerization with BF3/H2O
		Friedel-Crafts polymerization used in condensation polymerization reactions
done		Free radical polymerization by atom transfer

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Living polymerization processes refers to which of the following types of polymerization?

-

1

+

שאלה #38978

		Polymerization in biological systems.
done		Chain reaction polymerization, with an initiation, propagation step, but without ot a minimal termination step.
		Redox polymerzation in water.
		Chain reaction polymerization with an initiation, propagation with random termination steps.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Mechanical properties measured by the tensile modulus (stress/strain) as a function of temperature which of the following statements is correct? (Choose from the following four possibilities)

-

1

+

שאלה #38979

		Increasing temperature above the Tg but below the Tm, the tensile modules decreases strongly for both amorphous and crystalline polymers
		Above the Tg the modules for amorphous polymers drops to a plateau and stays constant until the Tm where it falls sharply again
		Crystalline polymers show a constant decline in the modules in the temperature range between the Tg and Tm
done		Increasing temperature above the Tg for crystalline polymers the tensile modules stays relatively unchanged until the Tm after which the modules drops sharply.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Mixtures of polymers are called blends. Which of the following is a true about the difference between miscible and non miscible blends? (Choose from the following four possibilities) .

-

1

+

שאלה #38980

		Miscible blends have two different Tg for each polymer and the non miscible blend one Tg.
		Miscible blends are characterized by a two phases and non-miscible blends by one phases.
		Additives are needed to reduce interfacial energy in miscible blends and thus domain size and to prevent interface voids.
done		The free energy of mixing in a miscible blend is negative and in a non miscible blend positive.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Natural rubber, Hevea is sticky, elastic and a mechanically weak polymer while Gutta-percha is harder and more non-elastic. Both are made of exactly the same monomer material (isoprene) and have the same chemical composition. What is the best explanation for their different properties?

-

1

+

שאלה #38981

		There is a large difference in molecular weight.
		Gutta-percha i is partially crosslinked.
		The stereochemistry of Hevea is atactic while Gutta-percha is isotactic allowing for better packing and is thus partially crystalline.
done		Hevea has cis-double bond geometry, and Gutta-percha has a trans-double bond geometry allowing for better packing and is thus partially crystalline.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Nylon can be made from the ring opening polymerization of a lactam (molecular weight 148) or the condefisation polymerization of an amino alkyl carboxylic acid (molecular weight 198), [fin both cases the resulting polyamide nylon has a number average molecular weight of 45,000 what is the average degree of polymerization DP of each polymer. (Choose from the following four possibilities)

-

1

+

שאלה #38982

done		For the lactam polymer- DP=304. and for the aminoalkycarboxlic acid polymer- DP= 250
		For the lactam polymer-DP 250, and for the aminoalkycarboxlic acid polymer- DP =304
		For the lactam polymer- DP=346, and for the aminoalkycarboxlic acid polymer-DP=250
		For the lactam polymer-DP= 346 and for the aminoalkycarboxlic acid polymer-DP=227

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Nylon 6 can be made from the ring opening polymerization of caprolactam (molecular weight 113) or the condensation polymerization of 6 —aminohexanoic acid (molecular weight 131). If in both cases the nylon 6 has a number average molecular weight of 21,000 what is the average degree of polymerization DP of each polymer. (Choose from the following four possibilities)

-

1

+

שאלה #38983

		For the Caprolactam polymer- DP=186, and for the 6-amimohexanoic acid polymer- DP= 160
done		For the Caprolactam polymer-DP 186, and for the 6-aminohexanoic acid polymer- DP =186
		For the Caprolactam polymer- DP=160, and for the 6-aminohexanoic acid polymer- DP=160
		For the Caprolactam polymer-DP= 93, and for the 6-aminohexanoic acid polymer- DP=160

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

Of the following statements on polymer molecular parameters affecting chain flexibility and thus Tg which one is not correct: (Choose from the following four possibilities)

-

1

+

שאלה #38984

		Chain flexibility is mainly determined by the flexibility of the main chain.
		Chain flexibility is not only determined by the groups present in the main chain-the side groups can also be important because of steric and secondary interactions.
		Since segment motion implies the concerted movement of many chain atoms and bulky 7 side chains which hinder rotation about the single bonds increase Tg.
done		It is expected that that polymers with high polarity or high cohesive energy density willhave a lower Tg than nonpolar polymers.

• gavel
• mood_bad
• share

הגב

מיין לפי דירוג חדש ישן

• «
• 1
• 2
• 3
• 4
• 5
• 6
• »