Question

Choose one of the following options that correctly fills the blanks labelled as I and II in the paragraph below.

Lower critical solution temperature (LCST) polymers exhibit a decrease in their water solubility with an increase in temperature. This happens because, at low temperatures, the hydrogen bonding between the I segments of polymer chain and the water molecules dominates. At high temperatures, the interactions between the II segments dominate.

• A. I-hydrophilic; II-hydrophobic
• B. I-hydrophobic; II-hydrophilic
• C. I-hydrophilic; II-polar
• D. I-hydrophobic; II-polar

Hint:

Below the LCST, water-loving groups keep the chain dissolved; above it, water-hating groups take over and the chain collapses.

Answer 1

The Correct Option is A

Step 1: Recall what LCST behavior means.
A polymer with a lower critical solution temperature (LCST) is soluble in water below its LCST and becomes insoluble (it collapses and phase-separates) above its LCST. Poly(N-isopropylacrylamide), or PNIPAM, is the classic example.

Step 2: Think about which interaction dominates at low temperature.
Below the LCST, the polymer chain stays dissolved in water. Solubility in water needs favorable interactions with water molecules, and those come from hydrogen bonding between water and the polar, hydrophilic parts of the polymer chain (such as amide or hydroxyl groups).
So segment I, the one whose hydrogen bonding with water dominates at low temperature, must be the hydrophilic segment.

Step 3: Think about what happens as temperature rises.
Raising the temperature makes it entropically costly to keep water ordered around the hydrophobic parts of the chain. Above the LCST, hydrophobic segments (such as isopropyl groups) start clustering with each other instead of staying exposed to water, so the chain collapses and precipitates out.
So segment II, the one whose interactions dominate at high temperature, must be the hydrophobic segment.

Step 4: Check the remaining options.
Option (B) reverses I and II, which would mean the polymer gets more soluble at high temperature, the opposite of LCST behavior.
Options (C) and (D) use "polar" for segment II, but the transition is specifically a hydrophilic-to-hydrophobic dominance switch, not a change in polarity, so these do not fit the standard LCST explanation.

Final Answer:
I is hydrophilic (dominates at low temperature) and II is hydrophobic (dominates at high temperature).
\[ \boxed{\text{I-hydrophilic; II-hydrophobic}} \]